README: Add instructions for PyCharm (#81)

Instructions to add `black` to "External Tools" in PyCharm.

Adapted from https://kirankoduru.github.io/python/pylint-with-pycharm.html

README.md

@@ -308,6 +308,34 @@ you should tell Flake8 to ignore these warnings.
 Use [proofit404/blacken](https://github.com/proofit404/blacken).
 
 
+### PyCharm
+
+1. Install `black`.
+
+        $ pip install black
+
+2. Locate your `black` installation folder.
+
+        $ which black         # MacOS/Linux
+        /usr/local/bin/black  # possible location
+
+       <!-->
+
+        $ where black         # Windows
+        %LocalAppData%\Programs\Python\Python36-32\Scripts\black.exe  # possible location
+        
+3. Open External tools in PyCharm with `File -> Settings -> Tools -> External Tools`.
+
+4. Click the + icon to add a new external tool with the following values:
+    - Name: Black
+    - Description: Black is the uncompromising Python code formatter.
+    - Program: <install_location_from_step_2>
+    - Arguments: $FilePath$
+
+5. Format the currently opened file by selecting `Tools -> External Tools -> black`.
+    - Alternatively, you can set a keyboard shortcut by navigating to `Preferences -> Keymap`.
+
+
 ### Vim
 
 Commands and shortcuts: