added some what is math stuff
last paragraph
Merge branch 'f-howtolearnmath' into develop
Pay our dues to LYAH.

I guess this accidentally got removed. Even if LYAH leaves a bit to be desired,
it is the motivation and namesake for the book, so we have to mention it.
Merge branch 'develop' into f-lyahcite
Move LYAH stanza to acknowledgements section
Expand on injective functions.
Have Travis bot only tell us if a build status changes.
Add subset content.
Merge pull request #83 from wei2912/travis-bot-shut-up

Get the Travis bot to shut up about useless messages
Merge pull request #84 from jesyspa/develop

Wrote sections on injective functions and subsets
Merge pull request #72 from beingbrown/develop

Add a "How to learn math" section
Merge pull request #75 from pharpend/f-lyahcite

Pay our dues to LYAH.
Merge branch 'develop' of github.com:learnmath/lysa into develop