Update instructions to install latest version of black

doc/CONTRIBUTORS.md

@@ -99,14 +99,18 @@ code needs re-formatting.
 Black can be installed using pip.
 
 ```bash
-pip install black
+pip install git+git://github.com/psf/black
 ```  
 
 To format the code, you run `black` followed by the filename you wish to reformat.  For formatting a particular file name filename.py.
 
 ```bash
 black filename.py
 ```
+You can run Black as a package if running it as a script doesn't work:
+```bash
+python -m black filename.py
+```
 
 In many cases, it will make your life easier if you only run black on
 files you've changed because you won't have to scroll through a pile of

requirements.txt

@@ -9,4 +9,5 @@ pyyaml
 pytest
 pytest-xdist
 pytest-cov
-pytest-asyncio
+pytest-asyncio
+git+git://github.com/psf/black