doc: add README

README.md

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+# matrix-notifier
+
+This is a simple to send a message to a Matrix room. It automatically logs in
+and sends the message when invoked. This was written to be used a notification
+script in my CI/CD pipelines.
+
+## How to use
+
+You need to define the following environment variables for the script to be 
+executed correctly:
+
+* `USER`: the user to login as.
+* `PASS`: the password to login with.
+* `ADDRESS`: the address of the homeserver to connect to.
+* `ROOM`: the room id, as can be found in the room parameters.
+* `MESSAGE`: the message you wish to send to the room.
+
+### Example
+
+```sh
+export ADDRESS='https://matrix.org'
+export USER='username'
+export PASS='password'
+export ROOM='!aaaaaaaaaaaaaaaaaa:matrix.org'
+export MESSAGE='This is my test message'
+./matrix-notifier
+```
+
+## How to run/install
+
+This script is packaged with `Nix`, you can just use `nix run .` to run it.
+
+The only dependencies are `bash`, `curl`, and `jq`, install those and you should
+be ready to go!