Adds git-add -p instructions

pages/common/git-add.md

@@ -4,7 +4,7 @@
 
 - Add a file to the index:
 
-`git add {{PATHSPEC}}`
+`git add {{path/to/file}}`
 
 - Add all files (tracked and untracked):
 
@@ -17,3 +17,7 @@
 - Also add ignored files:
 
 `git add -f`
+
+- Add parts of a file interactively:
+
+`git add -p {{path/to/file}}`