provision: Make instructions to source venv more obvious.

(This cleans up the print formatting a bit in tools/provision
and adds an explicit step in README.md.)

tools/provision

@@ -77,9 +77,20 @@ the Python version this command is executed with."""
     else:
         warnings.warn("Your Python version does not support mypy. `tools/run-mypy` will fail.")
 
-    print("{green}Success!{end_format}  Run\n    {bold}source '{activate}'{end_format}\nto activate virtualenv.".format(
-        green='\033[92m', bold='\033[1m', end_format='\033[0m',
-        activate=os.path.join(base_dir, venv_dir, venv_exec_dir, 'activate')))
+    green = '\033[92m'
+    end_format = '\033[0m'
+    bold = '\033[1m' 
+
+    print('\n')
+    print(green + 'Success!' + end_format)
+
+    activate_command = os.path.join(base_dir,
+                                    venv_dir,
+                                    venv_exec_dir,
+                                    'activate')
+    print('\nNow, run this!:')
+    print(bold + 'source ' + activate_command + end_format)
+
 
 if __name__ == '__main__':
     main()