(docs): Add docs on fetching contents via HTTP server (#166)

* (docs): Add docs on fetching contents via HTTP server

Signed-off-by: Bryce Palmer <bpalmer@redhat.com>

* add curl examples

Signed-off-by: Bryce Palmer <everettraven@gmail.com>

---------

Signed-off-by: Bryce Palmer <bpalmer@redhat.com>
Signed-off-by: Bryce Palmer <everettraven@gmail.com>

docs/fetching-catalog-contents.md

@@ -0,0 +1,42 @@
+# Fetching `Catalog` contents from the Catalogd HTTP Server
+This document covers how to fetch the contents for a `Catalog` from the
+Catalogd HTTP Server that runs when the `HTTPServer` feature-gate is enabled
+(enabled by default).
+
+All `Catalog`s currently have their contents served via the following endpoint pattern:
+`http://{httpServerBaseUrl}/catalogs/{Catalog.Name}/all.json`
+
+All responses will be a JSON stream where each JSON object is a File-Based Catalog (FBC)
+object.
+
+For example purposes we make the following assumption:
+- A `Catalog` named `operatorhubio` has been created and successfully unpacked
+(denoted in the `Catalog.Status`)
+
+## On cluster
+
+When making a request for the contents of the `operatorhubio` `Catalog` from within
+the cluster issue a HTTP `GET` request to 
+`http://catalogd-catalogserver.catalogd-system.svc/catalogs/operatorhubio/all.json`
+
+An example command to run a `Pod` to `curl` the catalog contents:
+```sh
+kubectl run fetcher --image=curlimages/curl:latest -- curl http://catalogd-catalogserver.catalogd-system.svc/catalogs/operatorhubio/all.json
+```
+
+## Off cluster
+
+When making a request for the contents of the `operatorhubio` `Catalog` from outside
+the cluster, we have to perform an extra step:
+1. Port forward the `catalogd-catalogserver` service in the `catalogd-system` namespace:
+```sh
+kubectl -n catalogd-system port-forward svc/catalogd-catalogserver <port>:80
+```
+
+Once the service has been successfully forwarded to a localhost port, issue a HTTP `GET`
+request to `http://localhost:<port>/catalogs/operatorhubio/all.json`
+
+An example `curl` request that assumes the port-forwarding is mapped to port 8080 on the local machine:
+```sh
+curl http://localhost:8080/catalogs/operatorhubio/all.json
+```