Document how to build tools in parallel (-j)

This significantly reduces build time.

Change-Id: Ic8f49f4a521a1fa1e4aaad593740d3aeacf408f0

GettingStarted.md

diff --git a/GettingStarted.md b/GettingStarted.md
index 9cc0711..c2de88c 100644
--- a/GettingStarted.md
+++ b/GettingStarted.md
@@ -64,7 +64,7 @@
 Install the tools used for development:
 
 ```bash
-m tools
+m tools -j$(nproc)
 ```
 
 ## Day-to-day Development Workflow