Add .editorconfig

This encodes the indentation style for libdrm and can be used with
various editors. See http://editorconfig.org for instructions.

Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>

.editorconfig

@@ -0,0 +1,19 @@
+# To use this config with your editor, follow the instructions at:
+# http://editorconfig.org
+
+root = true
+
+[*]
+charset = utf-8
+insert_final_newline = true
+
+[*.{c,h}]
+indent_style = space
+indent_size = 4
+
+[{Makefile.*,*.mk}]
+indent_style = tab
+
+[*.m4]
+indent_style = space
+indent_size = 2