Added documentation to solve exception during macro expansion errors

[umbreak]

Added documentation to solve java.lang.ClassNotFoundException when adding a custom predicate inside a project (not REPL).

This addresses the issue #448

[codecov]

Codecov Report

Merging #451 into master will increase coverage by 1.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #451      +/-   ##
==========================================
+ Coverage   96.96%   98.03%   +1.06%     
==========================================
  Files          56       57       +1     
  Lines         659      660       +1     
  Branches       14       14              
==========================================
+ Hits          639      647       +8     
+ Misses         20       13       -7

Impacted Files	Coverage Δ
...in/scala-2.12/eu/timepit/refined/api/Refined.scala	0% <0%> (ø)
...main/scala/eu/timepit/refined/scalaz/package.scala	100% <0%> (+72.72%)	⬆️

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[fthomas]

LGTM. Many thanks @umbreak!