Added README.md.

README.md

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+# mastoreddit
+
+You can mostly figure out how this works by looking at the config file.
+
+```
+{
+  "token": "put-your-token-here",
+  "hostname": "alpines.club",
+  "subreddit": "snowboarding",
+  "top_posts": 5,
+  "min_score": 100,
+  "skip_titles": ["daily discussion"]
+}
+```
+
+Set `MR_CONFIG` to be the path to the config file and `MR_RECENTS` to be the
+path to another writable file to store a little bit of data about which posts
+it's made recently.
+
+Then you can have it run periodically with a systemd timer to search for new hot
+posts and relay them to the bot.