Add a tips/tricks/FAQ section to the documentation.

This is just a very bare bones beginning to this chapter.
But it opens the door to actually adding content to this
one tip/trick at a time.

Change-Id: I2aa386ba4da66dba5308e3f4ed4d434e44e44fd9

docs/source/index.rst

diff --git a/docs/source/index.rst b/docs/source/index.rst
index 49ee63d..74ce20b 100644
--- a/docs/source/index.rst
+++ b/docs/source/index.rst
@@ -43,6 +43,7 @@
    tutorial
    modeling
    solving
+   tricks
    reading
    contributing
    acknowledgements

docs/source/tricks.rst

diff --git a/docs/source/tricks.rst b/docs/source/tricks.rst
new file mode 100644
index 0000000..c53ba02
--- /dev/null
+++ b/docs/source/tricks.rst
@@ -0,0 +1,17 @@
+.. _chapter-tricks:
+
+===================
+Tips, Tricks & FAQs
+===================
+
+A collection of miscellanous tips, tricks and frequently asked
+questions
+
+Derivatives
+===========
+
+The single most important bit of advice for users of Ceres Solver is
+to use analytic/automatic differentiation when you can. It is tempting
+to take the easy way out and use numeric differentiation. This is a
+bad idea. Numeric differentiation is slow, ill-behaved, hard to get
+right and results in poor convergence behaviour.