Create "How to get contribute code" page

[nmalky]

Elements:

• How to set up github
• How to choose something to work on
• Encouragement to make small changes
• How to make a small change (e.g. to website / docs) using GitHub online editor: all the steps. Make a commit to a new branch, submit a PR. In detail, for people who have never used git.
• How to set up git on your local machine
• Using git by CLI / GitHub desktop app (e.g. adding some code)
• Conventions for PRs, commits, comments

More advanced:

• Code review (?)
• Setting up dev environment

[nmalky]

This lacks the non-coding elements. Should perhaps be "How to contribute code"

[nmalky]

Now to be handled by #52