Document how to build tests in README (#1244)

Summary:
- Document in the `README.md` how to build tests as part of the CMake build.

Closes https://github.com/facebook/folly/issues/1240
Pull Request resolved: https://github.com/facebook/folly/pull/1244

Differential Revision: D17927395

Pulled By: yfeldblum

fbshipit-source-id: 6b638ac807886b505b69063f2a47162f3f3fc8de

README.md

@@ -104,6 +104,11 @@ cmake \
   -DCMAKE_LIBRARY_PATH=/alt/lib/path1:/alt/lib/path2 ...
 ```
 
+#### Building tests
+
+By default, building the tests is disabled as part of the CMake `all` target.
+To build the tests, specify `-DBUILD_TESTS=ON` to CMake at configure time.
+
 #### Ubuntu 16.04 LTS
 
 The following packages are required (feel free to cut and paste the apt-get