Adding notes about customers

Although I've found one mod that seems to be able to do this, it had lots of other features that I just don't need, so I'd rather not have too many unneeded things that I need to keep up with. My question is, how can I add notes to customers that can be viewed in admin side only.
For example, say if I need a note on that customer to let me know or remind me of specific things like, if there's certain special needs in services or maybe to record situations or issues in the past (like many late to no payments) or even just to say that this customer has certain favorite things or colors. Basically just an extra field that I can record information I need to know, for my own records, that will help me to help or deal with that customer accordingly.
Thanks in advance ^^
Oh, yes I'm currently using the latest version ( though I changed/updated the files it doesn't say the correct version o_o; )