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.

pkgs/README.md

@@ -132,6 +132,8 @@ Now that this is out of the way. To add a package to Nixpkgs:
 
     https://www.mozilla.org/en-US/firefox/55.0/releasenotes/
 
+(using "→" instead of "->" is also accepted)
+
 ## Category Hierarchy
 [categories]: #category-hierarchy