Feature/help fixes (#198)

* - Corrected variable names in README: cx_ast_client_id -> cx_client_id, cx_ast_client_secret -> cx_client_secret.
- (auth register) provided separat instructions for Linux and Windows, now there is only one set of instructions.

* - Small change to (auth register) help.

* - Cleaned text in README.

README.md

@@ -188,8 +188,8 @@ You can optionally configure the key/secret into your stored CLI configuration v
 You can also store the key/secret in the environment like this:
 
 ``` bash
-export CX_AST_CLIENT_ID=<your-client-id>
-export CX_AST_CLIENT_SECRET=<your-client-secret>
+export CX_CLIENT_ID=<your-client-id>
+export CX_CLIENT_SECRET=<your-client-secret>
 ./cx scan list
 ```