Document non-standard file prefixes in implementation-notes.md.

[pixelzoom]

Related to code review #32 ...

From the CRC:

• Do filenames use an appropriate prefix? ...

There are a number of files with "VSM" and "SM" prefixes. Non-standard prefixes should be documented somewhere, implemenation-notes.md looks like the intended place, but it's incomplete -- it identifies "VSM" but not "SM". I'm not sure if there are other such prefixes, because I'm partway through the review.

If you decide to keep common-vsm and common-sm in #197, you might also say something about them in implemenation-notes.md (correspondence to "VSM" and "SM", what value they are providing, etc.)

[matthew-blackman]

The commit looks good to me and adds the appropriate amount of documentation. Nice work! Closing.