Add an .editorconfig to ensure consistent whitespace policy

src/README.md already says:

    _Before_ beginning development, especially on teams, make sure to
    configure your linter and code formatting to conform to one
    unified setting

EditorConfig (see https://editorconfig.org/) is a fantastic way to
ensure that many editors will automatically configure themselves
with settings to correctly match any project containing an
.editorconfig file.  So add one to this project with settings matching
the existing style (including 2-column indentation).

Author: Adam Spiers

.editorconfig

@@ -0,0 +1,13 @@
+# EditorConfig  http://EditorConfig.org
+
+# top-most EditorConfig file
+root = true
+
+# All files
+[*]
+charset = utf-8
+end_of_line = lf
+indent_size = 2
+indent_style = space
+insert_final_newline = true
+trim_trailing_whitespace = true