Discuss extended formulations in docs

[ericphanson]

I'm not 100% sure this is the best place for it, but I wanted it somewhere people would see it fairly quickly, because I think it makes it more clear what exactly Convex.jl is doing and why DCP warnings are important. As I've worked on #393, I've realized the obvious fact that the central/only thing Convex.jl does is automatic application of extended formulations, and I didn't find that to be completely clear from the docs, so I wanted to write about it somewhere central.

The other reason for adding this is to make the issue with non-DCP problems more clear. I actually haven't seen too many issues where people are solving non-DCP problems and getting confused by the fact that the results are wrong, but I do think it can be confusing why that happens. For example, I thought for awhile that the main problem was it was a nonconvex problem and you are finding a local solution instead; really the issue is you are solving a completely different problem since the reformulation was invalid, and I hope the abs example makes that clear.

[ericphanson]

@matbesancon would you be up for reviewing this?

[codecov]

Codecov Report

Merging #396 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #396   +/-   ##
=======================================
  Coverage   88.42%   88.42%           
=======================================
  Files          75       75           
  Lines        3508     3508           
=======================================
  Hits         3102     3102           
  Misses        406      406           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update dd2f6c9...7fa2261. Read the comment docs.