[Docs Handbook] Add guideline on how to work on GitHub issues

It seems that we have developed a sort of a workflow for working on thsese issues but no one knows about it. Since we don't have PRs here we can't use some other GitHub features, such as "Request review" etc. Also, due to permissions, contributors can't assign themselves or move issues through our project boards.

So we have workarounds:

• You browse issues and find what you like. Post in comment that you'd like to work on it and start working.
• Someone will assign you and move issue into "In progress" column.
• When your work is ready for review, post another comment mentioning @zzap , @atachibana or someone else if you were discussing it with someone else.
• Someone will move issue to "Review" column. After review you'll be either asked to fix something or if you'd like to update documentation yourself.
• When documentation is updated the issue is moved to "Done" column.

Let's add it to README file here and our Handbook.