Document remove_column action

README.md

@@ -18,6 +18,7 @@ Reshape is an easy-to-use, zero-downtime schema migration tool for Postgres. It
 	- [Columns](#columns)
 		- [Add column](#add-column)
 		- [Alter column](#alter-column)
+		- [Remove column](#remove-column)
 	- [Indices](#indices)
 		- [Add index](#add-index)
 - [Commands and options](#commands-and-options)
@@ -275,6 +276,22 @@ down = "index - 1" # Decrement to revert for old schema
 
 ```
 
+#### Remove column
+
+The `remove_column` action will remove an existing column from a table. You can optionally provide a `down` setting. This should be an SQL expression which will be used to determine values for the old schema when inserting or updating rows using the new schema. The `down` setting must be provided when the removed column is `NOT NULL` or doesn't have a default value.
+
+*Example: remove column `name` from table `users`*
+
+```toml
+[[actions]]
+type = "remove_column"
+table = "users"
+column = "name"
+
+# Use a default value of "N/A" for the old schema when inserting/updating rows
+down = "'N/A'"
+```
+
 ### Indices
 
 #### Add index