Add CONTRIBUTING.md file

CONTRIBUTING.md

@@ -0,0 +1,42 @@
+# Contribution Guide
+
+## Developer Documentation
+### Regenerating Protobuf Source
+The project uses a git submodule under the [vendor directory](vendor/github.com/openconfig/gnmi) to keep proto files in
+sync with upstream. You can update the module and regenerate the related source using `make`.
+
+```sh
+make update
+```
+
+### gNMI Target Server
+You can use the [google/gnxi/gnmi_target](https://github.com/google/gnxi/tree/master/gnmi_target) to test the client 
+code. You can set this up using the the following commands (assuming you have ``golang` and `GOPATH` configured 
+correctly).
+
+#### Installation
+```sh
+# install the shell binary  
+go get -u github.com/google/gnxi/gnmi_target
+go install -v github.com/google/gnxi/gnmi_target
+```
+#### Sample Configuration
+You can use the configuration provided at [tests/integration/fixtures/config.json](tests/integration/fixtures/config.json) 
+or fetch it using curl.
+
+```sh
+# fetch test configuration from github
+curl -sLO https://raw.githubusercontent.com/python-gnxi/python-gnmi-proto/master/tests/integration/fixtures/config.json
+```
+
+Once you have a configuration you can start the server like shown below.
+```sh
+# run server
+gnmi_target \
+    -bind_address ":9339" \
+    -username admin \
+    -password secret \
+    -logtostderr \
+    -notls \
+    -config tests/integration/fixtures/config.json
+```