Merge #13718: docs: Specify preferred Python string formatting technique

b1b8863fd6 docs: Specify preferred Python string formatting technique (Mason Simon)

Pull request description:

  @practicalswift this documents the nit you suggested in my previous PR
  https://github.com/bitcoin/bitcoin/pull/13707#discussion_r203628395

Tree-SHA512: b75335976b30ccbce5c2e3a46e5b6218e797944e7025c17b4e56f294670397d731e1a9f4fe7e809d907136e2e62cd9c2b8404b875b4879a1e3f0b1970b94d5a8

test/functional/README.md

@@ -28,6 +28,7 @@ don't have test cases for.
 - When subclassing the BitcoinTestFramwork, place overrides for the
   `set_test_params()`, `add_options()` and `setup_xxxx()` methods at the top of
   the subclass, then locally-defined helper methods, then the `run_test()` method.
+- Use `'{}'.format(x)` for string formatting, not `'%s' % x`.
 
 #### Naming guidelines