RFR: Section on cloning

Fri Jun 11 00:30:13 UTC 2021

Added a section on cloning from GitHub, and an example of building on Mac. Also fixed two minor bugs that caused problems with syntax highlighting in Atom.

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Commit messages:
 - Section on cloning

Changes: https://git.openjdk.java.net/guide/pull/59/files
 Webrev: https://webrevs.openjdk.java.net/?repo=guide&pr=59&range=00
  Stats: 38 lines in 1 file changed: 29 ins; 4 del; 5 mod
  Patch: https://git.openjdk.java.net/guide/pull/59.diff
  Fetch: git fetch https://git.openjdk.java.net/guide pull/59/head:pull/59

PR: https://git.openjdk.java.net/guide/pull/59