Bug 1361708 - Update docs in readme.md; r=Honza

MozReview-Commit-ID: 3WPJCabyMNw

--HG--
extra : rebase_source : c93923a50e5ca273f9da8557452994b7a5e47bff

devtools/client/netmonitor/README.md

@@ -21,7 +21,9 @@ $ npm install -g yarn
 
 ## Quick Setup
 
-Navigate to the `netmonitor` folder with your terminal, then run the following commands:
+Navigate to the `mozilla-central/devtools/client/netmonitor` folder with your terminal.
+Note that this folder is available after `mozilla-central` was cloned in order to get
+a local copy of the repository. Then run the following commands:
 
 ```bash
 # Install packages