C++ quick start: add note about local installs (#691)

Closes #690

content/en/docs/languages/cpp/quickstart.md

@@ -104,7 +104,7 @@ While not mandatory, gRPC applications usually leverage [Protocol Buffers][pb]
 for service definitions and data serialization, and the example code uses
 [proto3][].
 
-The following commands will build and locally install gRPC and Protocol Buffers.
+The following commands build and locally install gRPC and Protocol Buffers:
 
 ```sh
 $ cd grpc
@@ -119,6 +119,12 @@ $ make install
 $ popd
 ```
 
+{{% alert title="Important" color="warning" %}}
+  We **strongly** encourage you to install gRPC _locally_ — using an
+  appropriately set `CMAKE_INSTALL_PREFIX` — because there is no easy way
+  to uninstall gRPC after you've installed it globally.
+{{% /alert %}}
+
 More information:
 
 - You can find a complete set of instructions for building gRPC C++ in [Building