Add note to issue template about FAQ

[freitagbr]

Fixes #787

Adds a note to the issue template that links to the FAQ.

[nfischer]

@freitagbr can we make this less specific? I imagine we'll expand our FAQ to include more stuff. Let's do something like "Before posting your issue, make sure to check our FAQ". LGTM otherwise.

[codecov-io]

Codecov Report

Merging #794 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #794   +/-   ##
======================================
  Coverage    95.5%   95.5%           
======================================
  Files          34      34           
  Lines        1269    1269           
======================================
  Hits         1212    1212           
  Misses         57      57

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[freitagbr]

@nfischer PTAL