Add documentation about new array syntax

doc/Developers-Intro.md

@@ -64,6 +64,7 @@ Here's a few primers if you are new to Friendica or to the PSR-2 coding standard
 * Boolean operators are `&&` and `||` for PHP conditions, `AND` and `OR` for SQL queries
 * No closing PHP tag
 * No trailing spaces
+* Array declarations use the new square brackets syntax
 
 Don't worry, you don't have to know by heart the PSR-2 coding standards to start contributing to Friendica.
 There are a few tools you can use to check or fix your files before you commit.