Documentation: Inform how to update the requirements lock file

Author: amotl

DEVELOP.rst

@@ -17,6 +17,9 @@ Then, run tests::
 
     $ bundle exec rspec
 
+Linter
+======
+
 Run linter::
 
     $ bundle exec rubocop
@@ -25,6 +28,13 @@ Update linter violations file::
 
     $ bundle exec rubocop --auto-gen-config
 
+Dependencies
+============
+
+Update lock file ``Gemfile.lock``::
+
+    $ bundle update
+
 
 Preparing a Release
 ===================