Add README.md to source dist.

As per this commit to resolve a similar issue
https://github.com/saily/python-memcached/commit/829fa09718ab75a6988eb7ef5857fd50bf942749
https://github.com/linsomniac/python-memcached/issues/5

Demonstrate the issue is resolved by running 'python setup.py sdist' (README.md is included)
```
(requestions)Vibur:src adamgriffiths$ python setup.py sdist
running sdist
running egg_info
creating requestions.egg-info
writing requirements to requestions.egg-info/requires.txt
writing requestions.egg-info/PKG-INFO
writing top-level names to requestions.egg-info/top_level.txt
writing dependency_links to requestions.egg-info/dependency_links.txt
writing manifest file 'requestions.egg-info/SOURCES.txt'
reading manifest file 'requestions.egg-info/SOURCES.txt'
reading manifest template 'MANIFEST.in'
warning: no files found matching '*.rst'
warning: no files found matching '*.txt'
warning: no files found matching 'ChangeLog'
warning: no files found matching 'MakeFile'
warning: no previously-included files matching '*.pyc' found anywhere in distribution
warning: no previously-included files matching '.gitignore' found anywhere in distribution
warning: no previously-included files matching '.DS_Store' found anywhere in distribution
writing manifest file 'requestions.egg-info/SOURCES.txt'
warning: sdist: standard file not found: should have one of README, README.txt

creating requestions-0.0.4
creating requestions-0.0.4/requestions
creating requestions-0.0.4/requestions.egg-info
making hard links in requestions-0.0.4...
hard linking MANIFEST.in -> requestions-0.0.4
hard linking README.md -> requestions-0.0.4
hard linking setup.py -> requestions-0.0.4
hard linking requestions/__init__.py -> requestions-0.0.4/requestions
hard linking requestions/httpetrified.py -> requestions-0.0.4/requestions
hard linking requestions/io.py -> requestions-0.0.4/requestions
hard linking requestions/utils.py -> requestions-0.0.4/requestions
hard linking requestions.egg-info/PKG-INFO -> requestions-0.0.4/requestions.egg-info
hard linking requestions.egg-info/SOURCES.txt -> requestions-0.0.4/requestions.egg-info
hard linking requestions.egg-info/dependency_links.txt -> requestions-0.0.4/requestions.egg-info
hard linking requestions.egg-info/not-zip-safe -> requestions-0.0.4/requestions.egg-info
hard linking requestions.egg-info/requires.txt -> requestions-0.0.4/requestions.egg-info
hard linking requestions.egg-info/top_level.txt -> requestions-0.0.4/requestions.egg-info
Writing requestions-0.0.4/setup.cfg
creating dist
Creating tar archive
removing 'requestions-0.0.4' (and everything under it)
(requestions)Vibur:src adamgriffiths$
```


Current tar ball on pypi
```
(requestions)Vibur:src adamgriffiths$ ls -R ~/Downloads/requestions-0.0.4/
PKG-INFO		requestions.egg-info	setup.py
requestions		setup.cfg

/Users/adamgriffiths/Downloads/requestions-0.0.4//requestions:
__init__.py	httpetrified.py	io.py		utils.py

/Users/adamgriffiths/Downloads/requestions-0.0.4//requestions.egg-info:
PKG-INFO		dependency_links.txt	requires.txt
SOURCES.txt		not-zip-safe		top_level.txt
```

tar ball after this commit
```
(requestions)Vibur:src adamgriffiths$ ls -R dist/requestions-0.0.4/
MANIFEST.in		README.md		requestions.egg-info	setup.py
PKG-INFO		requestions		setup.cfg

dist/requestions-0.0.4//requestions:
__init__.py	httpetrified.py	io.py		utils.py

dist/requestions-0.0.4//requestions.egg-info:
PKG-INFO		dependency_links.txt	requires.txt
SOURCES.txt		not-zip-safe		top_level.txt
```