Write up FlipperClient API.

Summary:
Specifies the API for implementing your own Flipper Client.
I believe this provides everything you need to know to build a flipper-compliant client.
The only snag is that the current certificate exchange procedure would require a change to allow for new OS's, device types etc. It would be much better if we had a TLS-opt out so you don't need to go through all that rigmarole.

Reviewed By: passy

Differential Revision: D15150550

fbshipit-source-id: 2878dcc174807c3f86c0562660e8a3e43e5777b1

website/i18n/en.json

@@ -26,7 +26,7 @@
         "title": "Error Handling"
       },
       "extending/establishing-a-connection": {
-        "title": "Establishing a connection"
+        "title": "Secure Communication"
       },
       "extending/index": {
         "title": "Clients and Plugins"
@@ -34,6 +34,9 @@
       "extending/js-setup": {
         "title": "JavaScript Setup"
       },
+      "extending/new-clients": {
+        "title": "Implementing a Flipper Client"
+      },
       "extending/send-data": {
         "title": "Sending Data to Plugins"
       },
@@ -126,6 +129,7 @@
       "Advanced Usage": "Advanced Usage",
       "Extending Flipper": "Extending Flipper",
       "Plugin Development": "Plugin Development",
+      "Other Platforms": "Other Platforms",
       "Internals": "Internals"
     }
   },