Updated Linux instructions (markdown)

Linux-instructions.md

@@ -1,3 +1,7 @@
+# **IMPORTANT**
+
+Starting from v5, Fira Code does not distribute OTF. All packages here that have `otf-*` in the name are **now outdated** and should be replaced with ttf equivalents.
+
 ## Installing with a Package Manager
 
 ### Ubuntu Zesty (17.04), Debian Stretch (9) or newer