RFR: 8265531: doc/building.md should mention homebrew install freetype

[George Adams]

Updates the docs/building.html guide to include steps to install FreeType on macOS using homebrew.

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Commit messages:
 - add macOS freetype install steps to docs/building.html

Changes: https://git.openjdk.java.net/jdk/pull/3590/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3590&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265531
  Stats: 2 lines in 2 files changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3590.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3590/head:pull/3590

PR: https://git.openjdk.java.net/jdk/pull/3590