documentation for grpcio (pantsbuild#7155)

### Problem

Documentation for grpcio generation tool

src/docs/common_tasks/BUILD

@@ -132,3 +132,9 @@ page(
   name='thrift_gen',
   source='thrift_gen.md',
 )
+
+
+page(
+  name='python_proto_gen',
+  source='python_proto_gen.md',
+)

src/docs/common_tasks/python_proto_gen.md

@@ -0,0 +1,50 @@
+# Python gRPC + protobufs
+
+## Problem
+You have `.proto` files defining protobufs and grpc services and you want Pants to generate Python code from them that you can use from a Python application.
+
+## Solution
+Create `python_grpc_library` targets and use the gen goal to generate code from the `.proto` files. There is a codegen task grpcio-run, that uses Python's grpcio library https://grpc.io/ and generates python code from .proto files.
+
+## Usage
+
+in a `BUILD` file near your proto files, create a `python_grpcio_library` target with your protos as a `sources`.
+
+```build
+python_grpcio_library(
+  sources=['service.proto'],
+  dependencies=[
+    '3rdparty/python:protobuf',
+  ]
+)
+```
+
+Then, you can add a dependency on this target in your python binary's `BUILD` file `dependencies` section:
+
+```build
+python_binary(
+  source='server.py',
+  dependencies=[
+#    [...]
+    'examples/src/protobuf/org/pantsbuild/example/grpcio/service'
+  ],
+)
+```
+
+## Example:
+An example Python grpc client/server can be found in [/examples/src/python/example/grpcio](https://github.com/pantsbuild/pants/tree/master/examples/src/python/example/grpcio)
+
+to create a gRPC server execute
+```bash
+./pants run examples/src/python/example/grpcio/server
+```
+
+and when server is running, run client example:
+```bash
+./pants run examples/src/python/example/grpcio/client
+```
+
+generated code can be found as usual in pants output directory:
+```bash
+./pants.d/gen/grpcio-run/current/examples.src.protobuf.org.pantsbuild.example.service.service/current/org/pantsbuild/example/service
+```

src/docs/docsite.json

@@ -96,6 +96,7 @@
     "test_suite": "dist/markdown/html/src/docs/common_tasks/test_suite.html",
     "thrift_deps": "dist/markdown/html/examples/src/thrift/org/pantsbuild/example/README.html",
     "thrift_gen": "dist/markdown/html/src/docs/common_tasks/thrift_gen.html",
+    "grpcio_gen": "dist/markdown/html/src/docs/common_tasks/python_proto_gen.html",
     "tshoot": "dist/markdown/html/src/docs/tshoot.html",
     "why_use_pants": "dist/markdown/html/src/docs/why_use_pants.html"
   },
@@ -173,6 +174,7 @@
        {"heading": "Code & Doc Generation"},
        {"pages" : [
                     "thrift_deps",
+                    "grpcio_gen",
                     "page"
                 ]
        },