grpc_cli: add info about cmake gRPC_BUILD_TESTS flag

To get grpc_cli make target generated with cmake, `gRPC_BUILD_TESTS` flag has to be defined. This is currently not specified in docs.

doc/command_line_tool.md

@@ -52,13 +52,26 @@ Mac systems with Homebrew:
 brew install gflags
 ```
 
-Once the prerequisites are satisfied, you can build the command line tool with
-the command:
+Once the prerequisites are satisfied, generate makefiles:
+
+```
+$ mkdir -p cmake/build
+$ cd cmake/build
+$ cmake -DgRPC_BUILD_TESTS=ON ../..
+```
+
+Finally you can build the command line tool with the command:
 
 ```
 $ make grpc_cli
 ```
 
+To speed up compilation time on linux, you can use make with following flag:
+
+```
+$ make grpc_cli -j$(nproc)
+```
+
 The main file can be found at
 https://github.com/grpc/grpc/blob/master/test/cpp/util/grpc_cli.cc