editorconfig: add .adoc extension

editorconfig: add .adoc extension

The .adoc extension is commonly used for AsciiDoc files.  In a future
commit, we'll update some files to switch from the .txt extension to the
.adoc extension, so update the EditorConfig file to use the same
configuration for both extensions, since we want the files to be
formatted completely identically whether they're using the older or
newer extension.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>