Add CONTRIBUTING rules and .editorconfig

In this merge request:

• Add a CONTRIBUTING with some code style rules and the contributor convenant (just common sense).
• Add a .editorconfig file with tab indent rules
• Update .gitignore

.editorconfig

+root = true
+
+[*]
+end_of_line = lf
+insert_final_newline = true
+charset = utf-8
+
+[*.{c,h}]
+indent_style = tab
+# tab_width = 4
+
+[Makefile]
+indent_style = tab
+
+[*.yml]
+indent_style = space
+indent_size = 2