RFR: 7903693 Add contribution guidelines section

[Ludvig Janiuk]

Adding in `building.md` as it seems the place one would read if making changes.

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Commit messages:
 - 7903693 Add contribution guidelines section

Changes: https://git.openjdk.org/jtreg/pull/192/files
 Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=192&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903693
  Stats: 13 lines in 1 file changed: 13 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jtreg/pull/192.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/192/head:pull/192

PR: https://git.openjdk.org/jtreg/pull/192