[skip changelog] Mention Boards Manager installation option in platform specification intro (#797)

Previously, manual installation alone was specifically described, which might cause new platform authors to think the specification is only for manually installed platforms. Boards Manager was only mentioned in passing, deep within the document.

Author: per1234

docs/platform-specification.md

@@ -1,6 +1,6 @@
 This specification is a 3rd party hardware format to be used in Arduino development software starting from the Arduino IDE 1.5.x series.<br>
-This specification allows a 3rd party vendor/maintainer to add support for new boards to the Arduino development software by providing a file to unzip into the *hardware* folder of Arduino's sketchbook folder.<br>
-It is also possible to add new 3rd party boards by providing just one configuration file.
+Platforms add support for new boards to the Arduino development software. They are installable either via [Boards Manager](package_index_json-specification.md) or manual installation to the *hardware* folder of Arduino's sketchbook folder.<br>
+A platform may consist of as little as a single configuration file.
 
 ## Hardware Folders structure