Tweak readme.md (#66)

Co-authored-by: Ethan Arrowood <[email protected]>

readme.md

@@ -10,6 +10,13 @@ $ npm install tsd
 ```
 
 
+## Overview
+
+This tool lets you write tests for your type definitions (i.e. your `.d.ts` files) by creating files with the `.test-d.ts` extension.
+
+These `.test-d.ts` files will not be executed, and not even compiled in the standard way. Instead, these files will be parsed for special constructs such as `expectError<Foo>(bar)` and then statically analyzed against your type definitions.
+
+
 ## Usage
 
 Let's assume we wrote a `index.d.ts` type definition for our concat module.