Merge pull request #5630 from yang-g/gflags_mention

Put back the gflags/gtest requirement to doc

CONTRIBUTING.md

@@ -21,6 +21,15 @@ In order to run all of the tests we provide, you will need valgrind and clang.
 More specifically, under debian, you will need the package libc++-dev to
 properly run all the tests.
 
+Compiling and running grpc C++ tests depend on protobuf 3.0.0, gtest and gflags.
+Although gflags is provided in third_party, you will need to manually install
+that dependency on your system to run these tests. Under a Debian or Ubuntu
+system, you can install the gtests and gflags packages using apt-get:
+
+```sh
+ $ [sudo] apt-get install libgflags-dev libgtest-dev
+```
+
 If you are planning to work on any of the languages other than C and C++, you
 will also need their appropriate development environments.