doc: Add plan to add selective OAuth/password filling-in depending on which was entered

TODO.md

@@ -1,4 +1,5 @@
 # To-do
 - Report that "ReadableConfig" error, which was super unhelpful and very annoying when you don't know why it's bugging.
-- Find a better notation for the error-handling in setup() – although the custom error message is desirable, it's very bad
-to have to implement it using such a ridiculously long line...
+- Find a better notation for the error-handling in read_config_template() – although the custom error message is desirable, it's very bad
+to have to implement it using such a ridiculously long line...
+- Refactor so as to allow *either* OAuth2 or password to be filled in, depending on which was entered (with preference for OAuth2).