Mark shears.sh as an executable

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
shears: Enhance the documentation substantially

This was suggested by Thomas Braun.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
shears: add an example

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
shears: branch names can also contain underscores

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Fix DOS line endings

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>