Update README.md: Add a "cd FirefoxReality" (Igalia#1031)

This makes it clear that the rest of the instructions need to be executed inside the cloned directory. E.g. the third_party directory goes inside the FirefoxReality directory at the root.

Author: mstange

README.md

@@ -25,6 +25,7 @@ The L10n wiki page can be found [here](https://github.com/MozillaReality/Firefox
 
 ```bash
 git clone [email protected]:MozillaReality/FirefoxReality.git
+cd FirefoxReality
 ```
 
 *Clone the third-party repo.*