chore(cspp): Include markdown file referenced in previous commits

oops.

cspp/static/usage.md

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+# Collaborative Sh💩t Posting Pipeline (CSPP)
+
+## Usage
+
+As a user of the pipeline, you simply need to perform an authenticated upload of an image with a caption. From there it will be processed and sent to the channel configured by the `cspp` service.
+
+The `image_path` is the path to the image on the local file system. The `caption` is the text that will be sent with the image to the channel.
+
+### Curl example
+
+To use this via `curl` you can do the following:
+
+```shell
+curl -X POST \
+  -F "image=@/path/to/file" \
+  -F "caption=String you want to with the picture"  \
+  -H "X-API-KEY: $API_KEY" \
+ <service URI>
+```
+
+### Other tools
+
+The structure for the upload is as follows if you are using a different tool:
+
+Send the API key as a header `X-API-KEY`.
+
+The payload using HTTP POST on the `/api` end is:
+
+```json
+{
+  "image_path": "/path/to/image.jpg",
+  "caption": "This is a caption",
+}
+```
+
+### Error handling
+
+If you send an invaliad payload, the server may accept it, attempt to processes
+it and then fail. If that happens, your requeset is kept and moved aside for
+either manual debugging or manual processing. You may not receive an error if
+that happens because the upload succeeded, just not the processing.