merge libmaple docs ("leaflabs-docs") into ./docs

In the past, libample documentation was forked out of this repository
because the documentation had increased in scope. For the librambutan,
and the rambutan project in general, we will try to keep documentation
closer to the source code, so the librambutan-specific documentation
should live here. Other sections of leaflabs-docs will be culled in a
following commit.

This merge attempts to maintain history by using a subtree strategy.
Followed directions at:

    http://nuclearsquid.com/writings/subtree-merging-and-you/

Full history for files should be accessible using the "--follow" flag to
git log, eg:

    git log --follow docs/source/adc.rst

It should be possible to pull patches from leaflabs-docs with:

    git pull -s subtree leaflabs-docs master

... at least until the docs in this repository diverge significantly.

1 files changed, 30 insertions, 0 deletions

diff --git a/docs/source/spi.rst b/docs/source/spi.rst
new file mode 100644
index 0000000..13c4c59
--- /dev/null
+++ b/docs/source/spi.rst
@@ -0,0 +1,30 @@
+.. _spi:
+
+=====
+ SPI
+=====
+
+The Serial Peripheral Interface Bus (SPI) is a serial data transfer
+protocol useful for interacting with a wide variety of hardware
+peripherals.
+
+The public libmaple API for managing the SPI ports is the
+:ref:`HardwareSPI <lang-hardwarespi>` class.
+
+Recommended Reading
+-------------------
+
+* `Wikipedia Article on Serial Peripheral Interface Bus (SPI)
+  <http://en.wikipedia.org/wiki/Serial_Peripheral_Interface_Bus>`_
+
+* `Arduino reference on SPI
+  <http://arduino.cc/en/Reference/SPI>`_
+
+* `Hardcore SPI on Arduino <http://web.archive.org/web/20100522034122/http://klk64.com/arduino-spi/>`_ by kik64
+
+* ST Documentation:
+
+  * Reference Manual `RM0008
+    <http://www.st.com/web/en/resource/technical/document/reference_manual/CD00171190.pdf>`_
+    (PDF), Chapter 25, "Serial Peripheral Interface"
+