docs: write README

README.md

@@ -1,8 +1,97 @@
-# README
+# Mapeo Legacy Export Format
 
-## TODO:
+The Mapeo Legacy Export Format is a format for reading and writing legacy Mapeo data. It can be used to export data from Mapeo to be transformed for import into CoMapeo, or for some other purpose.
 
-- MLEFReader
-- automatically generated fields:
-  1. created_at -> rename to createdAt?
-  2. timestamp -> rename to updatedAt?
+## The file format
+
+`.mlef` files are just [ZIP files](<https://en.wikipedia.org/wiki/ZIP_(file_format)>) with a different extension.
+
+They contain two folders:
+
+- `docs/`, which contains all documents and their versions.
+- `media/`, which contains all media, such as photos.
+
+Documents and media are identified by a hex-encoded ID, such as `74cb70988a17b6e4`. They're nested in prefixed folders to help avoid extremely large directories. For example, `74cb70988a17b6e4` is placed in `74/74cb70988a17b6e4/`.
+
+All of a document's versions are in the same folder. The first version will be called `_.json` and subsequent versions will be named with their version number.
+
+Here's an example directory structure:
+
+```
+docs/
+├─ d2/
+│  └─ d2b6e3d72200c082/
+│     └─ _.json
+└─ 74/
+   └─ 74cb70988a17b6e4/
+      ├─ _.json
+      ├─ 54aef99c451b09d15f9a3e8f0059fe8d5a97ee2b2f0cb605e3f04153c5723012@0.json
+      └─ 54aef99c451b09d15f9a3e8f0059fe8d5a97ee2b2f0cb605e3f04153c5723012@4.json
+
+media/
+├─ original/
+│  ├─ 4b/
+│  │  └─ 4b04270213860542d3fb2d56275fe12f.jpg
+│  └─ 9e/
+│     └─ 9e657b791646d5c8e1214c4b020a9dd3.jpg
+├─ preview/
+│  ├─ 4b/
+│  │  └─ 4b04270213860542d3fb2d56275fe12f.jpg
+│  └─ 9e/
+│     └─ 9e657b791646d5c8e1214c4b020a9dd3.jpg
+└─ thumbnail/
+   ├─ 4b/
+   │  └─ 4b04270213860542d3fb2d56275fe12f.jpg
+   └─ 9e/
+      └─ 9e657b791646d5c8e1214c4b020a9dd3.jpg
+```
+
+## Writing data
+
+The `write` function takes a `kappa.db` folder as input, and the path of the output MLEF file. For example:
+
+```javascript
+import { write } from 'mapeo-legacy-export-format'
+
+const input = '/path/to/kappa.db'
+const output = '/path/to/exported.mlef'
+await write(input, output)
+```
+
+Refer to [the tests](https://github.com/digidem/mapeo-legacy-export-format/blob/main/test/writer.js) for a detailed example.
+
+## Reading data
+
+The `reader` function returns a Promise that resolves with a `Reader` instance. This instance lets you iterate over all documents and their versions. It also lets you fetch media. It should be closed when you're finished.
+
+```javascript
+import { reader } from 'mapeo-legacy-export-format'
+
+const mlefPath = '/path/to/exported.mlef'
+
+const r = await reader(mlefPath)
+
+for await (const document of r.documents()) {
+  console.log('Document ID:', document.id)
+  for (const documentVersion of document.versions) {
+    console.log(
+      'Document version (null if first version):',
+      documentVersion.version
+    )
+    console.log('Raw document:', documentVersion.document)
+    console.log('Hypercore metadata:', documentVersion.hypercoreMetadata)
+  }
+}
+
+for await (const mediaVariant of r.getMediaById('abc.jpg')) {
+  console.log(
+    'Media variant (such as "original" or "preview"):',
+    mediaVariant.variant
+  )
+  console.log('Bytes:', mediaVariant.data)
+}
+
+await r.close()
+```
+
+Refer to [the tests](https://github.com/digidem/mapeo-legacy-export-format/blob/main/test/reader.js) for a detailed example.