docs: ✏️ add casfs README

README.md

@@ -23,6 +23,7 @@ npm i memfs
 - `experimental` [`fs` to File System Access API adapter](./docs/fsa/fs-to-fsa.md)
 - `experimental` [File System Access API to `fs` adapter](./docs/fsa/fsa-to-fs.md)
 - `experimental` [`crudfs` a CRUD-like file system abstraction](./docs/crudfs/index.md)
+- `experimental` [`casfs` Content Addressable Storage file system abstraction](./docs/casfs/index.md)
 
 
 ## Demos

docs/casfs/index.md

@@ -0,0 +1,63 @@
+# `casfs`
+
+`casfs` is a Content Addressable Storage (CAS) abstraction over a file system.
+It has no folders nor files. Instead, it has *blobs* which are identified by their content.
+
+Essentially, it provides two main operations: `put` and `get`. The `put` operation
+takes a blob and stores it in the underlying file system and returns the blob's hash digest.
+The `get` operation takes a hash and returns the blob, which matches the hash digest, if it exists.
+
+
+## Usage
+
+
+### `casfs` on top of Node.js `fs` module
+
+`casfs` builds on top of [`crudfs`](../crudfs//index.md), and `crudfs`—in turn—builds on top of
+[File System Access API](../fsa/fs-to-fsa.md).
+
+```js
+import * as fs from 'fs';
+import { nodeToFsa } from 'memfs/lib/node-to-fsa';
+import { FsaCrud } from 'memfs/lib/fsa-to-crud';
+
+const fsa = nodeToFsa(fs, '/path/to/folder', {mode: 'readwrite'});
+const crud = new FsaCrud(fsa);
+const cas = new CrudCas(crud, {hash});
+```
+
+The `hash` is a function which computes a hash digest `string` from a `Uint8Array` blob.
+Here is how one could look like:
+
+```ts
+import { createHash } from 'crypto';
+
+const hash = async (blob: Uint8Array): Promise<string> => {
+  const shasum = createHash('sha1');
+  shasum.update(blob);
+  return shasum.digest('hex');
+};
+```
+
+Now that you have a `cas` instance, you can use it to `put` and `get` blobs.
+
+```js
+const blob = new Uint8Array([1, 2, 3]);
+
+const hash = await cas.put(blob);
+console.log(hash); // 9dc58b6d4e8eefb5a3c3e0c9f4a1a0b1b2b3b4b5
+
+const blob2 = await cas.get(hash);
+```
+
+You can also delete blobs:
+
+```js
+await cas.delete(hash);
+```
+
+And retrieve information about blobs:
+
+```js
+const info = await cas.info(hash);
+```