Add open-source and cross-platform to project tagline

Author: meichthys

README.md

@@ -2,7 +2,7 @@
 
 [English](https://github.com/TriliumNext/Notes/blob/master/README.md) | [Chinese](https://github.com/TriliumNext/Notes/blob/master/README-ZH_CN.md) | [Russian](https://github.com/TriliumNext/Notes/blob/master/README.ru.md) | [Japanese](https://github.com/TriliumNext/Notes/blob/master/README.ja.md) | [Italian](https://github.com/TriliumNext/Notes/blob/master/README.it.md)
 
-TriliumNext Notes is a hierarchical note taking application with focus on building large personal knowledge bases.
+TriliumNext Notes is an open-source, cross-platform hierarchical note taking application with focus on building large personal knowledge bases.
 
 See [screenshots](https://triliumnext.github.io/Docs/Wiki/screenshot-tour) for quick overview: