RFR: Section on cloning [v5]

[Jesper Wilhelmsson]

> 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.

Jesper Wilhelmsson has updated the pull request incrementally with one additional commit since the last revision:

  Updates after review

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Changes:
  - all: https://git.openjdk.java.net/guide/pull/59/files
  - new: https://git.openjdk.java.net/guide/pull/59/files/e2b4205d..a38a016a

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=guide&pr=59&range=04
 - incr: https://webrevs.openjdk.java.net/?repo=guide&pr=59&range=03-04

  Stats: 5 lines in 1 file changed: 0 ins; 2 del; 3 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