RFR: 7903658 Document build/make.sh

[Ludvig Janiuk]

Add a note in building.md about build/make.sh, explaning that you can use it to rebuild faster.

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Commit messages:
 - 7903658 Document build/make.sh

Changes: https://git.openjdk.org/jtreg/pull/182/files
 Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=182&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903658
  Stats: 5 lines in 1 file changed: 5 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jtreg/pull/182.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/182/head:pull/182

PR: https://git.openjdk.org/jtreg/pull/182