How to contribute improvements to the file hi-itrans.mim (used to input devanAgarI text with ibus)?

I will need to add/ alter three of lines, of which two are the following:
("Q" "॒") ; not in ITRANS Devanagari table
("W" "॑") ; not in ITRANS Devanagari table



I have never done this sort of thing before, so I will appreciate very specific pointers on how to proceed and what the process is like.