Fix an incorrect statement in philosophy about Towncrier only supporting RST

The reality is that it is Jinja2 template based and can produce Markdown as well. It just used to be poorly documented. I helped Hynek figure it out for the attrs project at some point.

Author: webknjaz

docs/philosophy.rst

@@ -37,7 +37,9 @@ None fully embodied scriv's philopsophy.
 Tools most similar to scriv:
 
 - `towncrier`_: built for Twisted, with some unusual specifics: fragment type
-  is the file extension, issue numbers in the file name.  Only .rst files.
+  is the file extension, issue numbers in the file name.  Defaults to using
+  ``.rst`` files, but can be configured to produce Markdown or any other
+  output format, provided enough configuration.
 
 - `blurb`_: built for CPython development, specific to their workflow: issue
   numbers from bugs.python.org, only .rst files.