Specify config for text editors

Summary: This is sort of documentation.

Test Plan:
Used the plugin for Notepad++ with default indent of 4 spaces.
Opened file from `src/`, verified that it indents 2 spaces.
Opened file from `externals/`, verified that it indents 4 spaces.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3865

.editorconfig

@@ -0,0 +1,27 @@
+; http://editorconfig.org/
+
+[*]
+indent_style = space
+indent_size = 2
+charset = utf-8
+trim_trailing_whitespace = true
+insert_final_newline = true
+
+[src/parser/__tests__/bundle/*]
+trim_trailing_whitespace = false
+insert_final_newline = false
+
+[src/parser/__tests__/diff/*]
+trim_trailing_whitespace = false
+insert_final_newline = false
+
+[src/parser/__tests__/patches/*]
+trim_trailing_whitespace = false
+insert_final_newline = false
+
+[externals/**]
+; Use editor default (possible autodetection).
+indent_style =
+indent_size =
+trim_trailing_whitespace = false
+insert_final_newline = false