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

Mon Feb 8 09:59:57 UTC 2021

On Mon, 8 Feb 2021 07:20:05 GMT, Andrea Cosentino <github.com+5106647+oscerd at openjdk.org> wrote:

> 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.

I can't open an issue on JIRA, since I don't have an account currently.

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