docs: add docs

README.md

@@ -5,6 +5,34 @@
 
 A command line tool converts tsc output to ESM modules.
 
+## What does it do?
+
+Assume you have file1.js and file2.js.
+
+```javascript
+// file1.js
+import {v2} from './file2';
+const f2 = import('./file2');
+
+// file2.js
+import {external} from '../extenal/file';
+import {v1} from './file1';
+const f1 = import('./file1');
+```
+
+esmify renames `*.js` to `*.mjs` and disambiguates import sources in the code.
+
+```javascript
+// file1.mjs
+import {v2} from './file2.mjs';
+const f2 = import('./file2.mjs');
+
+// file2.mjs
+import {external} from '../extenal/file.js';
+import {v1} from './file1.mjs';
+const f1 = import('./file1.mjs');
+```
+
 ## Usage
 
 ```