mention exceptions to the EAPI rule

We don't want people updating the EAPI value if they're just importing
a Gentoo ebuild, so note that exception.

BUG=chromium:340036
TEST=tried uploading an EAPI=3 ebuild in chromiumos-overlay
TEST=`./pre-upload_unittest.py` passes

Change-Id: I340f82688b718e5fda63180f045e5b55483a4d2c
Reviewed-on: https://chromium-review.googlesource.com/185063
Reviewed-by: Gilad Arnold <garnold@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
1 file changed