Detailed documentation for API + Service Clients

[pmohankumar]

As of today, big challenge for consumers of Poseidon before they ripe the benefits is lack of detailed documentation on "how-to-do" stuff.

Though we improved upon wiki on arch, design principles etc behind poseidon as well as we've a sample app to get started, we lack documentation around actual implementation details like (not comprehensive list)

1. How to make some query parameters optionals in API
2. Does poseidon support optional path parameters
3. How to avoid a hydra failure while returning an optional response
4. How to do file upload, download
5. How to pass multi value headers to service clients
6. How to talk to a HTTPS service from poseidon service client
7. How to map exceptions to a http response globally
8. How to add global and api-local filters
   ...

Some of these have been shared as gist to consumers outside flipkart (like HealthFace.in)
https://gist.github.com/pmohankumar/aa173878e0abf65edd0ec9f9a4509f20
https://gist.github.com/pmohankumar/282d41444a9510a8f94d13f79ed375a4
https://gist.github.com/pmohankumar/6f51f4a15be9f693ed702185e3099315
https://gist.github.com/pmohankumar/e5013adea0a7279a2231861f33f915c4
https://gist.github.com/pmohankumar/1e86f88b6845d074415f27ac9a183136

But we need to improve upon the documentation soon!

@shrey-garg , can you get someone to work on this once the sale fever subsidies.