precious-files.txt: new document proposing new precious file type

We have traditionally considered all ignored files to be expendable, but
users occasionally want ignored files that are not considered
expendable.  Add a design document covering how to split ignored files
into two types: 'trashable' (what all ignored files are currently
considered) and 'precious' (the new type of ignored file).

Helped-by: Sebastian Thiel <sebastian.thiel@icloud.com>
Signed-off-by: Elijah Newren <newren@gmail.com>