added instructions for PyCharm File Watcher setup (#418)

* added instructions for PyCharm File Watcher setup

With these steps, PyCharm will run black on every file save.

* Update README.md

README.md

@@ -619,11 +619,22 @@ $ where black
     - Name: Black
     - Description: Black is the uncompromising Python code formatter.
     - Program: <install_location_from_step_2>
-    - Arguments: $FilePath$
+    - 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 -> External Tools -> External Tools - Black`.
 
+6. Optionally, run Black on every file save:
+
+    1. Make sure you have the [File Watcher](https://plugins.jetbrains.com/plugin/7177-file-watchers) plugin installed.
+    2. Go to `Preferences -> Tools -> File Watchers` and click `+` to add a new watcher:
+        - Name: Black
+        - File type: Python
+        - Scope: Project Files
+        - Program: <install_location_from_step_2>
+        - Arguments: `$FilePath$`
+        - Output paths to refresh: `$FilePathRelativeToProjectRoot$`
+        - Working directory: `$ProjectFileDir$`
 
 ### Vim