markdown-include is GPL licensed.
Also, rewrite it as a block processor, so that it works correctly
inside indented blocks.
Signed-off-by: Anders Kaseorg <anders@zulip.com>
This commit modifies the text to:
* Use number 1 for all steps and let Markdown take care of the rest.
* Removes the line that says this webhook is "experimental". It isn't
anymore.