docs(CONVENTIONS.md): add guidelines for variable naming and string c…

…oncatenation

The new document outlines coding conventions, specifically recommending
the use of at least three-letter variable names and the use of
fmt.Sprintf for string concatenation.

CONVENTIONS.md

@@ -0,0 +1,2 @@
+- use at least three letters variable names
+- use fmt.Sprintf for string concatenation