Add description

Author: n4o847

README.md

@@ -1,8 +1,45 @@
 # compile-time-typescript
 
-## Usage
+## Install
 
+```shell
+$ git clone https://github.com/n4o847/compile-time-typescript
+$ cd compile-time-typescript
+$ npm install
 ```
+
+## Usage
+
+```shell
 $ echo -n "world" | npx ts-node src examples/hello.ts
 Hello, world!
 ```
+
+## Test
+
+```shell
+$ npm test
+```
+
+## How it works
+
+We can write a program like this:
+
+```typescript
+type Main<Input extends string> = `Hello, ${Input}\n`;
+export default Main;
+```
+
+Compile-time-typescript saves the program as `callee.ts` and creates `caller.ts`:
+
+```typescript
+import Main from './callee';
+type Input = "HERE COMES THE INPUT";
+type Output = Main<Input>;
+```
+
+So the program must have a default export of a generic type that takes a type parameter and constructs a string literal type.
+
+Then compile-time-typescript type-checks `caller.ts` and extracts the type information of `Output`.
+
+If `Output` is a string literal type, its content is printed. Otherwise, an error occurrs.