RFR: 7259: README.md should mention that the build script works for Mac OS

Wed May 26 19:52:17 UTC 2021

Found that the build.sh script works on macOS, too.

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Commit messages:
 - Update README.md

Changes: https://git.openjdk.java.net/jmc/pull/251/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=251&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7259
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jmc/pull/251.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/251/head:pull/251

PR: https://git.openjdk.java.net/jmc/pull/251