RFR: Polish README.md

[Johnny Lim]

This PR polishes the `README.md` file a bit.

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Commit messages:
 - Polish README.md

Changes: https://git.openjdk.org/jcstress/pull/131/files
 Webrev: https://webrevs.openjdk.org/?repo=jcstress&pr=131&range=00
  Stats: 7 lines in 1 file changed: 0 ins; 2 del; 5 mod
  Patch: https://git.openjdk.org/jcstress/pull/131.diff
  Fetch: git fetch https://git.openjdk.org/jcstress pull/131/head:pull/131

PR: https://git.openjdk.org/jcstress/pull/131