add use-<proj>-custom FOO for any commit/branch/tag

While it was possible before to build any branch of mpv/ffmpeg/etc by
just git checkout $whatever and then ./build, it got reset after
./rebuild or ./update etc.

Add use-mpv-custom and use-ffmpeg-custom which accept any
commit/branch/tag which will survive ./rebuild, ./update, etc.

This should make it slightly easier to build past versions of mpv.

While at it, also add use-libass-{master|custom}

1 files changed, 7 insertions, 2 deletions

diff --git a/README.rst b/README.rst
index 8a70fc0..d1aa4e7 100644
--- a/README.rst
+++ b/README.rst
@@ -181,8 +181,13 @@ And run ``./rebuild`` or similar. Use this to switch back to git master:
 
     ./use-mpv-master
 
-Likewise, you can use ``./use-ffmpeg-master`` and ``./use-ffmpeg-release`` to
-switch between git master and the latest FFmpeg release.
+Or this to switch to a custom tag/branch/commit FOO:
+
+    ./use-mpv-custom FOO
+
+Likewise, you can use ``./use-ffmpeg-master``, ``./use-ffmpeg-release`` or
+``./use-ffmpeg-custom BAR`` to switch between git master, the latest FFmpeg
+release, or to a custom tag/branch/commit BAR.
 
 Use on your own risk.