Add contract guidelines

[LHerskind]

Description

Adds an initial guideline for contract writing with references to tools and style-guide.

Closes #593.

Checklist:

• I have reviewed my diff in github, line by line.
• Every change is related to the PR description.
• I have linked this pull request to the issue(s) that it resolves.
• There are no unexpected formatting changes, superfluous debug logs, or commented-out code.
• The branch has been merged or rebased against the head of its merge target.
• I'm happy for the PR to be merged at the reviewer's next convenience.

[Maddiaa0]

lgtm