RFR: 7133: Update the build script help to include the installCore option

Mon Feb 8 09:59:56 UTC 2021

Noticed this: if you build jmc with build.sh script for the first time, you'll need to run the installCore command first and then all the command listed for testing and packaging it.

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Commit messages:
 - Updated the build instructions in case of build.sh usage

Changes: https://git.openjdk.java.net/jmc/pull/215/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=215&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7133
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jmc/pull/215.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/215/head:pull/215

PR: https://git.openjdk.java.net/jmc/pull/215