Add a getting started example with express.js

[lorisleiva]

Similarly to the existing getting-started-* examples, create an example that helps setting up a very simple express.js app using the Metaplex SDK.

• Create an express.js app with only one route that accepts a mint address in the request and returns the NFT object from the SDK as the response.
• Write a detailed README file that explains the steps to reproduce to achieve the created example. Look at the current READMEs for inspiration.