Update documentation for How to contribute (contributing.md )

[Willmish]

written by @Sophietn updating issue
Should cover information on current contributors
And who can contribute - GSF member orgs permission vs member of public
How they can contribute
eg. public can raise an issue, raise a PR?, start a discussion thread with questions
member orgs can join weekly meeting calendar invite link, contribute to PRs
who can merge PRs etc

[Sophietn]

I'll make a start on this one today

[Sophietn]

From #423: Update to docs: Release schedule - "at most once per month, as frequent as possible" to update in README.md and CONTRIBUTING.md

If we need a security release - we prioritise that.
Documentation release can happen anytime.