RFR: 7225: Add workflow badge to the readme

[Marcus Hirt]

Trying a PR to see if it works.

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Commit messages:
 - Trying relative path
 - Trying a PR
 - Experimenting
 - Making badges work
 - 7225: Add workflow badges to the readme

Changes: https://git.openjdk.java.net/jmc/pull/233/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=233&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7225
  Stats: 2 lines in 2 files changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jmc/pull/233.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/233/head:pull/233

PR: https://git.openjdk.java.net/jmc/pull/233