Merge pull request #1515 from pherl/changelog-beta3

Initial draft for changelog.

CHANGES.txt

+2016-05-10 version 3.0.0-beta-3 (C++/Java/Python/Ruby/Nano/Objective-C/C#/JavaScript)
+  General
+  * Supported Proto3 lite-runtime in C++/Java for mobile platforms.
+  * Any type now supports APIs to specify prefixes other than
+    type.googleapis.com
+
+  C++ (Beta)
+  * Improved hash maps. TODO(gpike)
+  * Arenas
+      - Several inlined methods in Arena were moved to out-of-line to improve
+        build performance and code size.
+      - Added SpaceAllocatedAndUsed() to report both space used and allocated
+      - Added convenient class UnsafeArenaAllocatedRepeatedPtrFieldBackInserter
+  * Any
+      - Allow custom type URL prefixes in Any packing.
+      - TextFormat now expand the Any type rather than printing bytes.
+  * Performance optimizations and various bug fixes.
+
+  Java (Beta)
+  * Introduced an ExperimentalApi annotation. Annotated APIs are experimental
+    and are subject to change in a backward incompatible way in future releases.
+  * Introduced zero-copy serialization as an ExperimentalApi
+      - Introduction of the `ByteOutput` interface. This is similar to
+        `OutputStream` but provides semantics for lazy writing (i.e. no
+        immediate copy required) of fields that are considered to be immutable.
+      - `ByteString` now supports writing to a `ByteOutput`, which will directly
+        expose the internals of the `ByteString` (i.e. `byte[]` or `ByteBuffer`)
+        to the `ByteOutput` without copying.
+      - `CodedOutputStream` now supports writing to a `ByteOutput`. `ByteString`
+        instances that are too large to fit in the internal buffer will be
+        (lazily) written to the `ByteOutput` directly.
+      - This allows applications using large `ByteString` fields to avoid
+        duplication of these fields entirely. Such an application can supply a
+        `ByteOutput` that chains together the chunks received from
+        `CodedOutputStream` before forwarding them onto the IO system.
+  * Other related changes to `CodedOutputStream`
+      - Additional use of `sun.misc.Unsafe` where possible to perform fast
+        access to `byte[]` and `ByteBuffer` values and avoiding unnecessary
+        range checking.
+      - `ByteBuffer`-backed `CodedOutputStream` now writes directly to the
+        `ByteBuffer` rather than to an intermediate array.
+  * Improved the performance for lite-runtime. TODO(dweis)
+  * Various bug fixes and small feature enhancement.
+      - Fixed stack overflow when in hashCode() for infinite recursive oneofs.
+      - Fixed the lazy field parsing in lite to merge rather than overwrite.
+      - TextFormat now supports reporting line/column numbers on errors.
+
+  Python (Beta)
+  * TODO(jieluo)
+
+  Objective-C (Beta)
+  * Proto comments now come over as HeaderDoc comments in the generated sources
+    so Xcode can pick them up and display them.
+  * The library headers have been updated to use HeaderDoc comments so Xcode can
+    pick them up and display them.
+  * The per message and per field overhead in both generated code and runtime
+    object sizes was reduced.
+  * Generated code now include deprecated annotations when the proto file
+    included them.
+
+  C# (Beta)
+  In general: some changes are breaking, which require regenerating messages.
+  Most user-written code will not be impacted *except* for the renaming of enum
+  values.
+
+  * Allow custom type URL prefixes in `Any` packing, and ignore them when
+    unpacking
+  * `protoc` is now in a separate NuGet package (Google.Protobuf.Tools)
+  * New option: `internal_access` to generate internal classes
+  * Enum values are now PascalCased, and if there's a prefix which matches the
+    name of the enum, that is removed (so an enum `COLOR` with a value
+    `COLOR_BLUE` would generate a value of just `Blue`). An option
+    (`legacy_enum_values`) is temporarily available to disable this, but the
+    option will be removed for GA.
+  * `json_name` option is now honored
+  * If group tags are encountered when parsing, they are validated more
+    thoroughly (although we don't support actual groups)
+  * NuGet dependencies are better specified
+  * Breaking: `Preconditions` is renamed to `ProtoPreconditions`
+  * Breaking: `GeneratedCodeInfo` is renamed to `GeneratedClrTypeInfo`
+  * `JsonFormatter` now allows writing to a `TextWriter`
+  * New interface, `ICustomDiagnosticMessage` to allow more compact
+    representations from `ToString`
+  * `CodedInputStream` and `CodedOutputStream` now implement `IDisposable`,
+    which simply disposes of the streams they were constructed with
+  * Map fields no longer support null values (in line with other languages)
+  * Improvements in JSON formatting and parsing
+
+  Javascript (Alpha)
+  * TODO(haberman)
+
+  C++/Java Lite (Alpha)
+    A new "lite" generator parameter was introduced in the protoc for C++ and
+    Java for Proto3 syntax messages. Example usage:
+
+     ./protoc --cpp_out=lite:$OUTPUT_PATH foo.proto
+
+    The protoc will treat the current input and all the transitive dependencies
+    as LITE. The same generator parameter must be used to generate the
+    dependencies.
+
+    In Proto3 syntax files, "optimized_for=LITE_RUNTIME" is no longer supported.
+
+
 2015-12-30 version 3.0.0-beta-2 (C++/Java/Python/Ruby/Nano/Objective-C/C#/JavaScript)
   General
   * Introduced a new language implementation: JavaScript.