Added .png files to .gitignore

Calligraphy generates diagrams based on the code, in PNG format.
Those should not be versioned.

.gitignore

@@ -32,6 +32,7 @@ doc
 deps
 _darcs
 *.pdf
+*.png
 *.sql
 *.ini
 *.toml
@@ -52,4 +53,4 @@ gargantext_profile_out.dot
 
 dev.jwk
 .psc-ide-port
-logs/
\ No newline at end of file
+logs/