RFR: 7903238: JMH: Add entry '.idea/' to file '.gitignore'

[Guoxiong Li]

Hi all,

I use `JetBrains IDEA` as the IDE to develop JMH.
So it is good to add the related project directory `.idea/` to the file `.gitignore`.

Thanks for taking the time to review.

Best Regards,
-- Guoxiong

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Commit messages:
 - CODETOOLS-7903238

Changes: https://git.openjdk.org/jmh/pull/76/files
 Webrev: https://webrevs.openjdk.org/?repo=jmh&pr=76&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903238
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jmh/pull/76.diff
  Fetch: git fetch https://git.openjdk.org/jmh pull/76/head:pull/76

PR: https://git.openjdk.org/jmh/pull/76