[RTM] Clean up documentation

[pavanramkumar]

Closes #234

• [x] update cheatsheet with links to approx grad/hess/loss formulas for probit
• [x] update tutorial codeblocks
• [x] remove multinomial option from readme
• [x] update installation instructions

[codecov-io]

Codecov Report

Merging #264 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #264   +/-   ##
=======================================
  Coverage   74.56%   74.56%           
=======================================
  Files           4        4           
  Lines         629      629           
  Branches      128      128           
=======================================
  Hits          469      469           
  Misses        125      125           
  Partials       35       35

Impacted Files	Coverage Δ
pyglmnet/pyglmnet.py	80.71% <ø> (ø)	:arrow_up:

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[jasmainak]

@pavanramkumar let me know when it's ready from your end.

[pavanramkumar]

@jasmainak there's still more to do here; i'll do these soon

• [ ] update cheatsheet with approx grad/hess/loss formulas for probit
• [ ] update tutorial codeblocks
• [ ] smooth over examples

[jasmainak]

@pavanramkumar sounds good. I turned your comment into a todo list

[jasmainak]

@pavanramkumar you need to rebase first. It might change the diff a bit

[jasmainak]

Thanks @pavanramkumar !