Add an article on derivatives

Change-Id: I4d3efbc38dc068035f30ff808dfdc7970408a010

docs/source/bibliography.rst

@@ -94,6 +94,10 @@ Bibliography
    Part II: Implementation and Experiments**, Management Science,
    20(5), 863-874, 1974.
 
+.. [Press] W. H. Press, S. A. Teukolsky, W. T. Vetterling
+   & B. P. Flannery, **Numerical Recipes**, Cambridge University
+   Press, 2007.
+
 .. [Ridders] C. J. F. Ridders, **Accurate computation of F'(x) and
    F'(x) F"(x)**, Advances in Engineering Software 4(2), 75-76, 1978.
 

docs/source/index.rst

@@ -23,6 +23,7 @@ reports and feature requests.
 
    installation
    tutorial
+   derivatives
    guide
    features
    faqs

docs/source/nnls_solving.rst

@@ -141,7 +141,7 @@ setting :member:`Solver::Options::trust_region_strategy_type`.
 Levenberg-Marquardt
 -------------------
 
-The Levenberg-Marquardt algorithm [Levenberg]_ [Marquardt]_ is the
+The Levenberg-Marquardt algorithm [Levenberg]_  [Marquardt]_ is the
 most popular algorithm for solving non-linear least squares problems.
 It was also the first trust region algorithm to be developed
 [Levenberg]_ [Marquardt]_. Ceres implements an exact step [Madsen]_