DOC: add bs4/lxml install note to README.rst

Author: cpcloud

README.rst

@@ -90,6 +90,12 @@ Optional dependencies
  * openpyxl version 1.6.1 or higher, for writing .xlsx files
  * xlrd >= 0.9.0
  * Needed for Excel I/O
+ * `lxml <http://lxml.de>`__, or `Beautiful Soup 4 <http://www.crummy.com/software/BeautifulSoup>`__: for reading HTML tables
+ * The differences between lxml and Beautiful Soup 4 are mostly speed (lxml
+ is faster), however sometimes Beautiful Soup returns what you might
+ intuitively expect. Both backends are implemented, so try them both to
+ see which one you like. They should return very similar results.
+ * Note that lxml requires Cython to build successfully
  * `boto <https://pypi.python.org/pypi/boto>`__: necessary for Amazon S3 access.