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:
* Removes the unnecessary screenshot. The UI is intuitive enough
and standalone instructions should suffice.
* Rearranges the instructions into 4 steps.
* Makes the wording more explicit.