Add more doc

README.rst

@@ -570,6 +570,9 @@ from a security perspective to stop arbitrary Matrix users spidering 'internal'
 URLs on your network.  At the very least we recommend that your loopback and
 RFC1918 IP addresses are blacklisted.
 
+This also requires the optional lxml and netaddr python dependencies to be
+installed.
+
 
 Password reset
 ==============

UPGRADE.rst

@@ -34,7 +34,7 @@ Upgrading to v0.15.0
 ====================
 
 If you want to use the new URL previewing API (/_matrix/media/r0/preview_url)
-then you have to explicitly enable it in the config and install the optional
+then you have to explicitly enable it in the config and update your dependencies
 dependencies.  See README.rst for details.