Add wiki contribution guidelines

[hauni97]

Summary

Contribution guidelines for the community should be in the README.md.
This may include style guidelines when editing existing content & adding content.

A full list of processes can be curated as the wiki grows...

Basic example

For instance

• It is essential when mentioning SGX to understand that it is a trademarked product.
• For formal correctness, it should always be accompanied with the term 'Intel' + '®' then 'SGX' added to result in 'Intel® SGX'

Motivation

This can make the review of PRs easier (save time) and add conciseness to the reader with overall quality improvements.

Feel free to suggest more ideas in comments below.