Add explainer text to sgr engine add

As an "impatient onboarder" it wasn't immediately obvious to me the
implications of `sgr engine add` prompting for a password.
I first suspected `sgr` wanted root and believed it was a sudo prompt.

It seems reasonable to add some explainer text here, so as to assuage
any worries/confusion as to which password is expected/how it gets used.

splitgraph/commandline/engine.py

@@ -202,7 +202,9 @@ def _update_bar(
     "--cap-add", help="Add kernel capabilities to the engine container", multiple=True, default=[]
 )
 @click.argument("name", default=DEFAULT_ENGINE)
-@click.password_option()
+@click.password_option(
+    prompt="Enter a password to protect your local engine. \nWill be saved to .sgconfig (usually ~/.splitgraph/.sgconfig)\nPassword"
+)
 def add_engine_c(
     image,
     port,