RFR: 6684: Adding CONTRIBUTING.md file

[Marcus Hirt]

6684: Adding CONTRIBUTING.md file

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Commits:
 - 47da1634: 6684: Adding CONTRIBUTING.md file

Changes: https://git.openjdk.java.net/jmc/pull/46/files
 Webrev: https://webrevs.openjdk.java.net/jmc/46/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JMC-6684
  Stats: 45 lines in 1 file changed: 45 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jmc/pull/46.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/46/head:pull/46

PR: https://git.openjdk.java.net/jmc/pull/46