RFR: 8125: Update the developer guide for latest Eclipse

[Marcus Hirt]

Updating the dev guide with a workaround for Eclipse currently redirecting to https for http URLs by default, which doesn't work well with our locally running jetty.

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Commit messages:
 - 8125: Update the developer guide for latest Eclipse

Changes: https://git.openjdk.org/jmc/pull/521/files
 Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=521&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-8125
  Stats: 7 lines in 1 file changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jmc/pull/521.diff
  Fetch: git fetch https://git.openjdk.org/jmc.git pull/521/head:pull/521

PR: https://git.openjdk.org/jmc/pull/521