Clarify building from Git sources

Explain that release archive directory names in the installation guide
must be replaced with the actual Git checkout directory.

Fixes #1209

Change-Id: I66f2246aacd3b5223dcc3792c2cf4670cd6248c5

docs/source/installation.rst

diff --git a/docs/source/installation.rst b/docs/source/installation.rst
index d911f8f..2329b0f 100644
--- a/docs/source/installation.rst
+++ b/docs/source/installation.rst
@@ -120,6 +120,10 @@
 
    git clone --recurse-submodules https://github.com/ceres-solver/ceres-solver
 
+The build instructions below use ``ceres-solver-2.2.0`` as the source
+directory because they assume a release archive. If you build from a Git
+checkout, replace it with the name of your checkout, usually ``ceres-solver``.
+
 
 .. _section-linux:
 

docs/source/version_history.rst

diff --git a/docs/source/version_history.rst b/docs/source/version_history.rst
index 50f8353..f6a9193 100644
--- a/docs/source/version_history.rst
+++ b/docs/source/version_history.rst
@@ -11,6 +11,15 @@
 Version History
 ===============
 
+Unreleased
+==========
+
+Bug Fixes & Minor Changes
+-------------------------
+
+#. Clarify building Ceres Solver from Git sources in the installation
+   documentation.
+
 2.2.0
 =====