docs: add semver section

[kentcdodds]

Addresses comments in #27

[kentcdodds]

May not be necessary to merge this until we setup auto-releases.

[jonathanglasmeyer]

On hold till we use auto-release.

[codecov-io]

Current coverage is 100.00%

Merging #33 into master will not affect coverage as of a7a0b37

@@            master     #33   diff @@
======================================
  Files           34      34       
  Stmts          264     264       
  Branches         0       0       
  Methods          0       0       
======================================
  Hit            264     264       
  Partial          0       0       
  Missed           0       0       

Review entire Coverage Diff as of a7a0b37

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