Add .editorconfig

This might consolidate whitespace usage across the project eventually.

Author: prusnak

.editorconfig

@@ -0,0 +1,13 @@
+# EditorConfig configuration for BitcoinDesign/Guide
+# https://EditorConfig.org
+
+# Top-most EditorConfig file
+root = true
+
+# Unix-style newlines with a newline ending every file, utf-8 charset
+[*]
+end_of_line = lf
+insert_final_newline = true
+trim_trailing_whitespace = true
+charset = utf-8
+indent_style = space