resolves #1 add contributing guidelines for specification issues and MRs

Merged Sarah White requested to merge swhitesal/asciidoc-lang:issue-1-contributing-guide into main

This MR describes the contributing guidelines and steps for people interested in submitting helpful specification issues and MRs.

It does not describe the guidelines for contributing to the end user documentation. That content should be discussed and provided in a separate issue/MR.

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