RFR: Added pandoc version recommendation

[Iris Clark]

On Wed, 16 Sep 2020 02:27:49 GMT, Jesper Wilhelmsson <jwilhelm at openjdk.org> wrote:

> 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.

Marked as reviewed by iris (Reviewer).

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