added CMake usage instructions

cmake/INSTALL.md

+Building Glog with CMake
+========================
+
+1. Create a build directory and `cd` to it.
+2. Run
+    ```bash
+    cmake path/to/glog
+    ```
+
+3. Afterwards, generated files (GNU make, Visual Studio, etc.) can be used to
+   compile the project.
+
+
+Consuming Glog in a CMake Project
+=================================
+
+To use Glog in your project `myproj`, use:
+
+```cmake
+cmake_minimum_required (VERSION 3.0)
+project (myproj)
+
+find_package (glog 0.3.4 REQUIRED)
+
+add_executable (myapp main.cpp)
+target_link_libraries (myapp glog::glog)
+```
+
+Compile definitions and options will be added automatically to your target as
+needed.