RFR: 7386: Remove reference to Fedora in the readme

[Alex Macdonald]

This PR addresses JMC-7386 [[0]](https://bugs.openjdk.java.net/browse/JMC-7386), in which the README should be updated to remove the reference to Red Hat packaging JMC for Fedora (as we don't do that anymore).

I've also added a bit of context for how JMC is available in RHEL 7 and RHEL 8.

[0] https://bugs.openjdk.java.net/browse/JMC-7386

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Commit messages:
 - 7386: Remove reference to Fedora in the readme

Changes: https://git.openjdk.java.net/jmc/pull/304/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=304&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7386
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jmc/pull/304.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/304/head:pull/304

PR: https://git.openjdk.java.net/jmc/pull/304