Add information on caching credentials in memory

src/configuration/user.md

@@ -10,4 +10,7 @@ git config --global user.name "Your Name"
 # Update individual repository user name / email.
 git config user.name "Your Name"
 git config user.email "your_email@example.com"
+
+# Store credentials in cache/memory (default 900 seconds/15 minutes).
+git config credential.helper cache
 ```