Update documentation

Author: peter-evans

README.md

@@ -87,6 +87,16 @@ This means that reactions to comments will appear to be made by the user account
           commands: deploy, integration-test, build-docs
 ```
 
+### How are comments parsed for slash commands?
+
+Slash commands must be placed in the first line of the comment to be interpreted as a command.
+
+- The first character must be a `/`
+- The slash command extends to the last non-whitespace character on the first line
+- Anything after the first line is ignored and can be freely used for comments
+
+![Comment Parsing](docs/assets/comment-parsing.png)
+
 ## Handling dispatched commands
 
 ### Event types