Less provocative introductory paragraph

Less provocative introductory paragraph

Some people interpret the previous wording to mean that Monocypher
already eats Libsodium's lunch, which is obviously false.  Status
slap-down ensues, and I have to explain the difference between "means to"
and "does".

This also hints more clearly at the scope of Monocypher: do no more than
TweetNacl (except for password key derivation), stay small and portable
without sacrificing too much speed. (That scope may need to be clearly
stated somewhere.)