Pattern docs

[elegantmoose]

Includes

• Pattern guide
• docstrings for patterns.py

Please read through guide, expecting requested edits and modifications.

Fixes #206.

[clenk]

Looks good; thanks @mbastian1135!

[codecov-io]

Codecov Report

Merging #207 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #207      +/-   ##
==========================================
- Coverage   99.51%   99.51%   -0.01%     
==========================================
  Files          62       62              
  Lines        6553     6551       -2     
==========================================
- Hits         6521     6519       -2     
  Misses         32       32

Impacted Files	Coverage Δ
stix2/test/test_pattern_expressions.py	100% <100%> (ø)	⬆️
stix2/patterns.py	99.3% <100%> (ø)	⬆️

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