Update SECURITY.md

Build Steps.

Author: branch-nchoudry

SECURITY.md

@@ -1,3 +1,14 @@
+#### How to build?
+1. Install Xcode
+2. Install cocoapods - https://cocoapods.org/ or <code>brew install cocoapods</code>
+3. Checkout https://github.com/BranchMetrics/ios-branch-deep-linking-attribution. Make sure you switch to the latest release tag.                     
+4. Change into the Branch-TestBed directory          
+5. Remove Podfile.lock
+6. Run <code>pod update</code>
+7. open Branch-TestBed.xcworkspace to run the test app.
+
+
+
 # Security Policy
 
 ## Reporting a Vulnerability