Fix README to be proper RST syntax

The README file was in RST, but had a formatting issue with the title.
The usage of the README is setup was incorrectly setting the content
type to markdown, rather than RST. This seems to have caused our twine
check to validate the package as fine, but PyPi apparently checks
package metadata a different way and rejected the package upload due to
issues with the README file.

This corrects the RST syntax, renames the file from txt to rst to
accurately reflect its content type, and updates the settings up
setup.py to correctly report the content as RST.

Change-Id: I809b3964d1f4d239ea60f953a7cdcbcc9c9a98ff
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

README.rst

@@ -1,5 +1,6 @@
+================
 XStatic-Dagre-D3
---------------
+================
 
 Dagre-D3 JavaScript library packaged for setuptools (easy_install) / pip.
 

setup.py

@@ -3,14 +3,14 @@ from xstatic.pkg import dagre_d3 as xs
 
 # The README.txt file should be written in reST so that PyPI can use
 # it to generate your project's PyPI page.
-long_description = open('README.txt').read()
+long_description = open('README.rst').read()
 
 setup(
     name=xs.PACKAGE_NAME,
     version=xs.PACKAGE_VERSION,
     description=xs.DESCRIPTION,
     long_description=long_description,
-    long_description_content_type="text/markdown",
+    long_description_content_type="text/x-rst",
     classifiers=xs.CLASSIFIERS,
     keywords=xs.KEYWORDS,
     maintainer=xs.MAINTAINER,