Improve FetchContent example in docs

* Provide a complete copy/pasteable CMakeLists.txt file
* Add link to install-prereqs docs

src/cpp/README.md

@@ -70,18 +70,26 @@ also sets up an `add_subdirectory()` rule for you. This causes gRPC to be
 built as part of your project.
 
 ```cmake
+cmake_minimum_required(VERSION 3.15)
+project(my_project)
+
 include(FetchContent)
 FetchContent_Declare(
   gRPC
   GIT_REPOSITORY https://github.com/grpc/grpc
   GIT_TAG        v1.25.0
 )
+set(FETCHCONTENT_QUIET OFF)
 FetchContent_MakeAvailable(gRPC)
 
 add_executable(my_exe my_exe.cc)
 target_link_libraries(my_exe grpc++)
 ```
 
+Note that you need to
+[install the prerequisites](../../BUILDING.md#pre-requisites)
+before building gRPC.
+
 ### git submodule
 If you cannot use FetchContent, another approach is to add the gRPC source tree
 to your project as a