Merge pull request #7 from badsyntax/readme

Update README with better usage instructions

README.md

@@ -18,11 +18,26 @@ npm install s3-etag
 ```ts
 import { generateETag } from 's3-etag';
 
+// Simple MD5 hash of contents for non-multipart files
 const etag = generateETag(absoluteFilePath);
 
-expect(etag).toBe('ba0b9fe20dc67b0e4c478ab3c03dc5ce');
+// MD5 hash of combined contents & part number (see below) for multipart files
+const partSizeInBytes = 10 * 1024 * 1024; // 10mb
+const etag = generateETag(absoluteFilePath, partSizeInBytes);
 ```
 
+## How It Works
+
+This is a Node.js implementation of [this algorithm](https://stackoverflow.com/a/19896823/492325).
+
+At a high level:
+
+- If no `partSizeInBytes` is specified, return MD5 hash of file contents
+- If `partSizeInBytes` is specified:
+  - Generate parts by comparing `partSizeInBytes` to the file size
+  - Read each part from the file, MD5 hash the part, and append it to a global combined hash
+  - Once all parts are processed, generate a new MD5 from the global combined hash, and suffix with the amount of parts
+
 ## License
 
 See [LICENSE.md](./LICENSE.md).

package.json

@@ -2,7 +2,7 @@
   "name": "s3-etag",
   "version": "0.0.0",
   "private": false,
-  "description": "Generate an S3 ETAG for multipart uploads in Node.js",
+  "description": "Generate an accurate S3 ETAG for multipart uploads in Node.js",
   "exports": "./index.js",
   "type": "module",
   "scripts": {