gitfaq: files in .gitignore are tracked

Add issue in 'Common Issues' section which addresses the problem of
Git tracking files/paths mentioned in '.gitignore'.

Signed-off-by: Shourya Shukla <shouryashukla.oo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitfaq.txt

@@ -223,6 +223,16 @@ a file checked into the repository which is a template or set of defaults which
 can then be copied alongside and modified as appropriate.  This second, modified
 file is usually ignored to prevent accidentally committing it.
 
+[[files-in-.gitignore-are-tracked]]
+I asked Git to ignore various files, yet they are still tracked::
+	A `gitignore` file ensures that certain file(s) which are not
+	tracked by Git remain untracked.  However, sometimes particular
+	file(s) may have been tracked before adding them into the
+	`.gitignore`, hence they still remain tracked.  To untrack and
+	ignore files/patterns, use `git rm --cached <file/pattern>`
+	and add a pattern to `.gitignore` that matches the <file>.
+	See linkgit:gitignore[5] for details.
+
 Hooks
 -----