Documentation & Logging cleanups.

1. Document the use of dogleg and a general discussion of
   trust region methods.
2. Added a TBD section on compiler/linker flags.
3. Summary::FullReport now prints out sparse_linear_algebra_library
   and trust_region_strategy_type.

Change-Id: I01f680070d510715900f345364855689005d54bb
diff --git a/docs/changes.tex b/docs/changes.tex
index 7fca3ee..4daaeee 100644
--- a/docs/changes.tex
+++ b/docs/changes.tex
@@ -1,6 +1,21 @@
 %!TEX root = ceres-solver.tex
 
 \chapter{Version History}
+\section*{1.2.1}
+\subsection{New Features}
+\begin{itemize}
+	\item Powell's Dogleg solver
+	\item Documentation now has a brief overview of Trust Region methods and how the Levenberg-Marquardt and Dogleg methods work.
+\end{itemize}
+\subsection{Bug Fixes}
+\begin{itemize}
+	\item Destructor for TrustRegionStrategy was not virtual (Thanks Markus Moll)
+	\item Invalid DCHECK in suitesparse.cc (Thanks Markus Moll)
+	\item Logging level changes in ConjugateGradientsSolver
+	\item VisibilityBasedPreconditioner setup does not account for skipped camera pairs. This was debugging code.
+	\item Enable SSE support on MacOS
+	\item \texttt{system\_test} was taking too long and too much memory (Thanks Koichi Akabe)
+\end{itemize}
 \section*{1.2.0}
 \subsection{New Features}
 \begin{itemize}