Add instructions for forking/cloning and raising a PR for Go code

Proposed change

On the getting started doc, write a complete worked example for cloning/forking and raising a PR with Go code. https://docs.openfaas.com/contributing/get-started/

Detail flow with example commands for one of the repos:

• Find issue
• Ask to work on it
• Fork repo
• Clone original repo
• Add new remote side
• Make a change
• Test it
• Sign-off commit
• Push to remote
• Raise PR
• Fill out template
• Reference PR on issue
• Tell us about it

Add a note on how to sign a commit / amend it if someone got this wrong.

Context

Suggested by @bartsmykla during review of How to Contribute presentation

Your Environment

• FaaS-CLI version ( Full output from: faas-cli version ):

• Docker version docker version (e.g. Docker 17.0.05 ):

• Are you using Docker Swarm or Kubernetes (FaaS-netes)?

• Operating System and version (e.g. Linux, Windows, MacOS):

• Link to your project or a code example to reproduce issue:

• Please also follow the troubleshooting guide and paste in any other diagnostic information you have:

Start with the getting started guide at https://docs.openfaas.com/contributing/get-started/. Add a complete Go example covering the listed fork, clone, remote, change, test, sign-off, push, pull request, and issue-reference steps, including how to correct a missing commit signature. Done means a newcomer can follow the example end to end.