Add README with draft docs for generic importers

Author: calumcalder

extensions/data-transfer/portability-data-transfer-generic/README.md

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+# Generic Importers
+
+## Implementing a Server
+
+To consume data from a generic importer, create a HTTP service in the language of your choice.
+The service should accept POST requests on a given base URL, and kebab-case sub-path specific to each data vertical (`https://example.com/import/blobs`, `https://example.com/import/social-posts`).
+
+### HTTP Interface
+
+Data will be POSTed to your endpoint with a `Content-Type` of either `application/json` for basic data-types, or `multipart/related` for file-based data-types. See below for how to interpret each of these.
+Your endpoint should return a status 201 if the resource has been created, 40x for errors caused by the Importer, or 50x for service errors.
+
+#### Basic data types
+
+For basic data-types the Importer will send a POST request with a `Content-Type: application/json` header. The body of the POST request will be a UTF-8 encoded JSON payload conforming to the relevant data-type schema detailed in the [Schemas](#schemas) section.
+
+For example, below is a full request of a SOCIAL_POST item (JSON formatted for readability)
+
+```http
+POST /import/social-posts HTTP/1.1
+Content-Type: application/json
+Content-Length: 510
+Host: localhost:8080
+Connection: Keep-Alive
+Accept-Encoding: gzip
+User-Agent: okhttp/3.9.1
+
+{
+  "@type": "SocialActivityData",
+  "metadata": {
+    "@type": "SocialActivityMetadata",
+    "actor": {
+      "@type": "SocialActivityActor",
+      "id": "321",
+      "name": "Steve",
+      "url": null
+    }
+  },
+  "activity": {
+    "@type": "SocialActivityModel",
+    "id": "456",
+    "published": 1731604863.845677,
+    "type": "NOTE",
+    "attachments": [
+      {
+        "@type": "SocialActivityAttachment",
+        "type": "IMAGE",
+        "url": "foo.com",
+        "name": "Foo",
+        "content": null
+      }
+    ],
+    "location": {
+      "@type": "SocialActivityLocation",
+      "name": "foo",
+      "longitude": 10.0,
+      "latitude": 10.0
+    },
+    "title": "Hello world!",
+    "content": "Hi there",
+    "url": null
+  }
+}
+```
+
+#### File-based data types
+
+To support large files as part of MEDIA, PHOTOS, VIDEOS and BLOBS imports, the request for these data-types is a `multipart/related` request with a `boundary`. \
+The first part will be an `application/json` payload containing metadata related to the file being importer. \
+The second part will have its `Content-Type` set to the MIME type of the file, and will be a multi-part stream of the bytes in the file with `boundary` separators.
+
+```http
+POST /import/blobs HTTP/1.1
+Content-Type: multipart/related; boundary=1581c5eb-05d0-42fe-bfb3-472151f366cd
+Content-Length: 524288384
+Host: localhost:8080
+Connection: Keep-Alive
+Accept-Encoding: gzip
+User-Agent: okhttp/3.9.1
+
+--1581c5eb-05d0-42fe-bfb3-472151f366cd
+Content-Type: application/json
+Content-Length: 130
+
+{"@type":"BlobbyFileData","folder":"/root/foo","document":{"name":"bar.mp4","dateModified":"2020-02-01","encodingFormat":"video/mp4"}}
+--1581c5eb-05d0-42fe-bfb3-472151f366cd
+Content-Type: video/mp4
+Content-Length: 524288000
+
+...524288000 bytes follow
+--1581c5eb-05d0-42fe-bfb3-472151f366cd--
+```
+
+> [!NOTE]
+> Endpoints supporting file-based data-types may also receive basic data-types, for example the `/blobs` endpoint should
+> support receiving folders encoded as `application/json` POST data.
+
+## Schemas
+
+```json
+{
+    "TODO": "schema"
+}
+```