Add sphinx modules needed for docs to PIP requirements file

Signed-off-by: Rick Altherr <kc8apf@kc8apf.net>

requirements.txt

diff --git a/requirements.txt b/requirements.txt
index 612ba38..f16b1bd 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,2 +1,5 @@
-yapf
 futures
+sphinx
+sphinx_rtd_theme
+sphinx-autobuild
+yapf