docs: Talk about how to use S3 alternatives

website/src/docs/aws-s3.md

@@ -171,6 +171,46 @@ uppy.use(AwsS3, {
 
 See the [aws-presigned-url example in the uppy repository](https://github.com/transloadit/uppy/tree/master/examples/aws-presigned-url) for a small example that implements both the server-side and the client-side.
 
+### S3 Alternatives
+
+Many other object storage providers have an identical API to S3, so you can use the AwsS3 plugin with them. You do have to provide a custom signing endpoint for these providers. You can use the official AWS SDK for the server-side language of your choice to create one of these endpoints fairly easily.
+
+For example, with DigitalOcean Spaces, you could use the client-side AwsS3 plugin configuration from the [previous example](#Generating-a-presigned-upload-URL-server-side), and create a signing endpoint using [express](https://expressjs.com) and the [aws-sdk](https://npmjs.com/package/aws-sdk) for Node.js:
+
+```js
+const S3 = require('aws-sdk/clients/s3')
+const app = require('express')()
+
+const s3 = new S3({
+  endpoint: 'YOUR_REGION.digitaloceanspaces.com',
+  accessKeyId: 'YOUR_ACCESS_KEY',
+  secretAccessKey: 'YOUR_SECRET_KEY'
+})
+
+app.post('/params', (req, res) => {
+  const { filename, contentType } = req.body
+
+  s3.getSignedUrl('putObject', {
+    Bucket: 'YOUR_SPACE_NAME',
+    Key: filename,
+    ContentType: contentType,
+    Expires: 5 * 60 * 1000 // 5 minutes
+  }, (err, data) => {
+    if (err) return next(err)
+
+    res.setHeader('content-type', 'application/json')
+    res.end(JSON.stringify({
+      method: 'put',
+      url: data
+    }, null, 2))
+  })
+})
+
+app.listen(8080)
+```
+
+For a working example that you can run and play around with, see the [digitalocean-spaces](https://github.com/transloadit/uppy/tree/master/examples/digitalocean-spaces) folder in the Uppy repository.
+
 ### Retrieving presign parameters of the uploaded file
 
 Once the file is uploaded, it's possible to retrieve the parameters that were