Make client/registerCapability a request (#353)

From the spec
> Register Capability
> The client/registerCapability request is sent from the server to the client to register for a new capability on the client side.

Updated Readme.md too!

Author: ManasJayanth

Readme.md

@@ -72,13 +72,11 @@ And you're done!
 
 1. Download the `your-platform.zip` file from the [latest release](https://github.com/jaredly/reason-language-server/releases), unzip it, and put the `reason-language-server.exe` file somewhere.
 2. Assuming you're using [lsp-mode](https://github.com/emacs-lsp/lsp-mode/), add the following to your config file:
-```
+```lisp
 (lsp-register-client
- (make-lsp-client :new-connection (lsp-stdio-connection "/absolute/path/to/reason-language-server.exe")
+  (make-lsp-client :new-connection (lsp-stdio-connection "/absolute/path/to/reason-language-server.exe")
                   :major-modes '(reason-mode)
-                  :notification-handlers (ht ("client/registerCapability" 'ignore))
-                  :priority 1
-                  :server-id 'reason-ls))
+                  :server-id 'ocaml-ls))
 ```
 
 ### Atom

src/lsp/Main.re

@@ -52,7 +52,7 @@ let getInitialState = (params) => {
   Log.log("Hello - from " ++ Sys.executable_name);
   Log.log("Previous log location: " ++ Log.initial_dest);
 
-  Rpc.sendNotification(
+  Rpc.sendRequest(
     Log.log,
     stdout,
     "client/registerCapability",