RFR: 8347033: add build instructions for iOS

[Abhinay Agarwal]

Adds build instruction for iOS

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Commit messages:
 - 8347033: add build instructions for iOS

Changes: https://git.openjdk.org/mobile/pull/34/files
  Webrev: https://webrevs.openjdk.org/?repo=mobile&pr=34&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8347033
  Stats: 70 lines in 1 file changed: 70 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/mobile/pull/34.diff
  Fetch: git fetch https://git.openjdk.org/mobile.git pull/34/head:pull/34

PR: https://git.openjdk.org/mobile/pull/34