V1 tag and deprecation (#240)

* deprecation warning

* Update message and CLI call

* Add to readme as well

hooks/pre_gen_project.py

@@ -0,0 +1,22 @@
+def deprecation_warning():
+    print("""
+
+=============================================================================
+*** DEPRECATION WARNING ***
+
+Cookiecutter data science is moving to v2 soon, which will entail using
+the command `ccds ...` rather than `cookiecutter ...`. The cookiecutter command
+will continue to work, and this version of the template will still be available.
+To use the legacy template, you will need to explicitly use `-c v1` to select it.
+
+Please update any scripts/automation you have to append the `-c v1` option,
+which is available now.
+
+For example:
+    cookiecutter -c v1 https://github.com/drivendata/cookiecutter-data-science
+=============================================================================
+
+    """)
+
+
+deprecation_warning()