Add NPM exec information

[MR4online]

I wish the Readme would explain how to run this lib via npm commands.
If I'm right, it's:

npm install -D apigen-ts
npm exec -- apigen-ts https://petstore3.swagger.io/api/v3/openapi.json ./api-client.ts

[vladkens]

Hi, @MR4online. If you use NPM you can run with npx, like: npx apigen-ts https://petstore3.swagger.io/api/v3/openapi.json ./api-client.ts.

[MR4online]

Thanks!
But anyway, it's missing in the readme of an npm package, which would make it easier for beginners.