add some documentation around the requirement for xcode cli tools on mac
diff --git a/docs/installation.rst b/docs/installation.rst
index 1c25ff7..c47e2d1 100644
--- a/docs/installation.rst
+++ b/docs/installation.rst
@@ -125,8 +125,21 @@
You should replace the version string on the first line as appropriate for your
build.
+Building cryptography on OS X
+-----------------------------
+
+To install the Xcode command line tools on OS X open a terminal window and run:
+
+.. code-block:: console
+
+ $ xcode-select --install
+
+This will install a compiler (clang) along with the required development
+headers. If you wish to compile against a more recent OpenSSL than the
+version shipped with OS X see the next section.
+
Using your own OpenSSL on OS X
-------------------------------
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
To link cryptography against a custom version of OpenSSL you'll need to set
``ARCHFLAGS``, ``LDFLAGS``, and ``CFLAGS``. OpenSSL can be installed via