fix: enable prettier for md

[jpoon]

What this PR does / why we need it:

Prettier supports MD (https://prettier.io/blog/2018/07/29/1.14.0.html#markdown). Using it to maintain consistency of our docs too

Which issue(s) this PR fixes

Special notes for your reviewer:

[TravisBuddy]

Hey @jpoon,
Something went wrong with the build.

TravisCI finished with status errored, which means the build failed because of something unrelated to the tests, such as a problem with a dependency or the build process itself.

[TravisBuddy]

Hey @jpoon,
Something went wrong with the build.

TravisCI finished with status errored, which means the build failed because of something unrelated to the tests, such as a problem with a dependency or the build process itself.