Update README.md

The README.md was woefully out of date, as it was advertising
Ceres Solver as just a non-linear least squares library.

Change-Id: I9a58fb8b63c08ee3852ffbae955e2ca8c90cd6d9

README.md

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-Ceres Solver - A non-linear least squares minimizer
-===================================================
+Ceres Solver - A large scale non-linear optimization library
+============================================================
+
+Ceres Solver is an open source C++ library for modeling and solving
+large, complicated optimization problems. It is a feature rich, mature
+and performant library which has been used in production at Google
+since 2010. Ceres Solver can solve two kinds of problems.
+
+1. Non-linear Least Squares problems with bounds constraints.
+2. General unconstrained optimization problems.
 
 Please see [ceres-solver.org](http://ceres-solver.org/) for more
 information.