Merge pull request #3129 from sigiesec/deprecate-msvc-sln-builds

Problem: prepared Visual Studio solution files are hard to maintain and redundant with CMake builds

builds/README

@@ -2,3 +2,13 @@ This directory holds build tools, i.e. tools we use to build the current
 code tree. Packaging tools (which take released tarballs or github code
 repos) should go into /packaging.
 
+Note: 'deprecated-msvc' contains deprecated prepared Visual Studio Solution 
+files for various Visual Studio versions. These are no longer maintained, 
+and may or may not work. Please use cmake instead to build with Visual
+Studio. Rationale: The solution and project files are hard to maintain,
+since there are different variants for each Visual Studio version.
+The tests have never been included there for this reason, so they were
+never usable for actual development of libzmq. If you encounter that 
+something that worked before does not work with CMake, please open as 
+issue at https://github.com/zeromq/libzmq/issues.
+