Introduce a check that links that source files with documentation about them

[wfarner]

This is preparation for #305, to help us keep the yet-to-be-written docs in sync with sources.

Signed-off-by: Bill Farner bill@docker.com

[codecov-io]

Current coverage is 68.97% (diff: 100%)

Merging #307 into master will increase coverage by 15.00%

@@             master       #307   diff @@
==========================================
  Files            10         27     +17   
  Lines           428       1441   +1013   
  Methods           0          0           
  Messages          0          0           
  Branches          0          0           
==========================================
+ Hits            231        994    +763   
- Misses          164        355    +191   
- Partials         33         92     +59   

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[chungers]

LGTM

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/docker/docker/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "doc-check" git@github.com:wfarner/infrakit.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842354643712
editor opens
change each 'pick' to 'edit'
save the file and quit
$ git commit --amend -s --no-edit
$ git rebase --continue # and repeat the amend for each commit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.