Documentation on Gateway Plugins implementation

[ndegwamartin]

We need some documentation on some of our technical implementation so that we can have an external reference for conversations with other stakeholders e.g. Clients, Google as well as help onboarding new engineers.

We need to document.

1. The way + why(challenges) we use it with the fhir-gateway as not as an external plugin as documented on the parent repo README but rather bundled the gateway as a library dependency (for the plugin). Mention also that a direct implication of this is that anytime one needs to use the latest HAPI FHIR Server, they would need a new release of the FHIR Gateway Plugins artifact.

2. The set up instructions for the gateway for external teams e.g. Download the image from our Dockerhub repo here https://hub.docker.com/r/onaio/fhir-gateway-plugin/tags and just run that.

3. Differences on how we access custom and non custom endpoints. You can copy from some of your new documentation here PR 19

Ideally, you need to document these on the FHIR Core docs here under Engineering (Create a new Backend section) https://docs.opensrp.io/ however a Google doc should be okay as well if short of time.

[rehammuzzamil]

Here is the documentation. For the complete documentation, readme.md has all the details. @ndegwamartin @dubdabasoduba