Update README.md

Handle large protected depths

Portals will now fail to ignite instead of defaulting to a fallback depth of (starty - 256) when no unprotected target can be found.
The search depth is also increased from (starty - 256) to (starty - 646)

This is to properly handle large protected areas that are sometimes set up at spawn - see issue minetest-mods#26

Retroactively tagging version 1.1

Fix minetest-mods#1.