README.md: mention that → can be used to signal version upgrades

Almost 1% of contributions in nixpkgs already use the unicode "→"
character instead of the two "->" ASCII characters to signal
a version upgrade. The guide only mentions "->", which sometimes
triggers discussions about the acceptable characters to use in
commit messages, with often a reference to the current README.md.

Some people prefer to use "→" because, depending on the font, it might
have a more appealing visual aspect than "->" and look better aligned.
Some others prefer to only write using the ASCII character set and will
use "->", but nowadays everyone can display common unicode characters
such as "→". Let us make everyone happy by indicating that both kind of
arrows are acceptable.