Add README

README.md

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+# Mastodon Image Bot
+
+This is a small program that posts an image from `./posts` every time you call it. You can call it like so:
+
+```
+MASTODON_INSTANCE_URL=... MASTODON_ACCESS_TOKEN=... deno run --allow-env --allow-net --allow-write=./posts --allow-read=./posts main.ts
+```