Add caveat to docs to discourage addition of new calculate blocks
Add caveat to docs to discourage addition of new calculate blocks

While this paragraph still offers useful advice, I don't want it to encourage
people to add *new* `calculate` blocks.

stwalsh/smart-answers,stwalsh/smart-answers,aledelcueto/smart-answers,alphagov/smart-answers,aledelcueto/smart-answers,aledelcueto/smart-answers,stwalsh/smart-answers,alphagov/smart-answers,alphagov/smart-answers,alphagov/smart-answers,aledelcueto/smart-answers,stwalsh/smart-answers