fix building of doc in virtualenv. add short description in readme

doc/Makefile

@@ -4,7 +4,7 @@
 # You can set these variables from the command line.
 PYTHON        ?= python
 SPHINXOPTS    ?=
-SPHINXBUILD   ?= sphinx-build
+SPHINXBUILD   ?= python $(shell which sphinx-build)
 PAPER         ?=
 
 # Internal variables.

doc/README.md

@@ -0,0 +1,5 @@
+# Building docs #
+To build docs, run `make` in this directory. `make help` lists all targets.
+
+## Requirements ##
+Sphinx is needed to build doc. Install with `pip install sphinx`.