Add deprecation notice and Blender compatibility notice to README

[Calinou]

See #132.

This closes godotengine/godot-proposals#2130.

[UndercoverPervert]

I've recently forked and made some tweaks/fixes to the exporter to be used with a conversion tool for a game I'm modding, I know this PR is supposed to announce the exporter as deprecated but I could PR my changes in case others find it useful, I've updated the colors export to RGBA, unflipped bitangent arrays (not sure if this is actual issue or an error with the conversion tool import process, but doing this fixed it, I could make this into a toggle though) and disabled bone renaming (the import tool depends on the bone names in order to match them to the game's skeleton, I could also make this into a toggle if needed)

[tetrapod00]

It makes sense to me to explicitly mark this as unmaintained, if it's not being maintained.