RFR: 8347033: [mobile] add ios-specific build docs [v4]

[Abhinay Agarwal]

> Adds build instruction for iOS

Abhinay Agarwal has updated the pull request incrementally with one additional commit since the last revision:

  move documentation to README.md

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Changes:
  - all: https://git.openjdk.org/mobile/pull/34/files
  - new: https://git.openjdk.org/mobile/pull/34/files/09bb3d2b..69e9d4f8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=mobile&pr=34&range=03
 - incr: https://webrevs.openjdk.org/?repo=mobile&pr=34&range=02-03

  Stats: 92 lines in 2 files changed: 45 ins; 47 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