chore: update contribution guide (#106)

CONTRIBUTING.md

@@ -3,8 +3,8 @@
 :heart: We love contributions of new CDNs, bug fixes, and improvements to the
 code.
 
-To add new domains or subdomains to an existing CDN, add them to `domains.json`
-or `subdomains.json` respectively.
+To add new domains, subdomains or paths to an existing CDN, add them to one or
+more of the JSON files in `data`.
 
 To add a new CDN, add the following:
 
@@ -14,20 +14,37 @@ To add a new CDN, add the following:
   that implements the `UrlGenerator` interface.
 - if the CDN should delegate remote source images to a different CDN where
   possible, implement `delegateUrl` and import it in `src/delegate.ts`. This is
-  likely to apply to all self-hosted image servers. See the `vercel` transformer
+  likely to apply to all self-hosted image servers. See the `ipx` transformer
   for an example.
 - a new test file in `src/transformers`. This should test all of the exported
-  API functions.
+  API functions,
 - at least one entry in `domains.json`, `subdomains.json` or `paths.json` to
-  detect the CDN. Do not include paths that are likely to cause false positives.
-  e.g. `/assets` is too generic, but `/_mycdn` is ok.
+  detect the CDN, unless it is not auto-detected. Do not include paths that are
+  likely to cause false positives. e.g. `/assets` is too generic, but `/_mycdn`
+  is ok.
 - add the new CDN to the types in `src/types.ts`
 - import the new source file in `src/transform.ts` and `src/parse.ts`
 - add a sample image to `examples.json` in the demo site. Run the site locally
-  to see that it works.
-- ensure tests pass by installing Deno and running `deno test src --allow-net`
+  to see that it works. This step is important! This is used for the e2e tests
+  to ensure that the transformer works as expected, so it needs a real image to
+  work with. Ideally this should be a public sample image used in the CDN's own
+  docs, but if that is not available then any image hosted on the CDN will do.
 
-### Image defaults
+## Testing
+
+This project uses [Deno](https://deno.com/) for development, so there is no
+install or build step. To run the tests you need to install Deno, then run:
+
+```sh
+deno test src --allow-net
+```
+
+This will run all of the unit tests and e2e tests.
+
+The playground site is in `demo`. To run it locally, run `yarn install` then
+`yarn dev` in the demo directory.
+
+## Image defaults
 
 When generating image URLs, we expect transformers to use the following defaults
 if supported, to ensure consistent behaviour across all CDNs:
@@ -41,3 +58,10 @@ if supported, to ensure consistent behaviour across all CDNs:
 - No upscaling. The image should not be upscaled if it is smaller than the
   requested dimensions. Instead it should return the largest available size, but
   maintain the requested aspect ratio.
+
+## Publishing
+
+The module is published to both [deno.land](https://deno.land/x/unpic) and
+[npm](https://www.npmjs.com/package/unpic), with the npm version generated using
+[dnt](https://github.com/denoland/dnt). This is handled automatically by GitHub
+Actions.