RFR: Added pandoc version recommendation

[Jesper Wilhelmsson]

It was noted that pandoc 1.12.3.1 did not support all the flags we use in the build. V2.0 has all flags we use so I
added a recommendation to use at least that version.

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Commit messages:
 - Added pandoc version recommendation

Changes: https://git.openjdk.java.net/guide/pull/27/files
 Webrev: https://webrevs.openjdk.java.net/?repo=guide&pr=27&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/guide/pull/27.diff
  Fetch: git fetch https://git.openjdk.java.net/guide pull/27/head:pull/27

PR: https://git.openjdk.java.net/guide/pull/27