Create documentation for bright object masks

The bright object mask code ported from hsc bought the ability to mask regions, during coaddition, by providing mask files. How to create these files, and where they should be placed in the file system is documented on and HSC ticket (https://hsc-jira.astro.princeton.edu/jira/browse/HSC-1351) but not on the LSST side. The information on how to create and use bright object masks should be put into LSST documentation.