Specify config for text editors

Test Plan: D3865

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3879

.editorconfig

@@ -0,0 +1,15 @@
+; http://editorconfig.org/
+
+[*]
+indent_style = space
+indent_size = 2
+charset = utf-8
+trim_trailing_whitespace = true
+insert_final_newline = true
+
+[externals/**]
+; Use editor default (possible autodetection).
+indent_style =
+indent_size =
+trim_trailing_whitespace = false
+insert_final_newline = false