* Adds C++ icon as submitted by @kareldonk
cplusplus.svg icon created by @kareldonk and
submitted as a zip file in issue #472. Format
corrected by @btrem to match other files in repo.
Also adds reference image and updates .md files.
* Reduces cplusplus.svg size
Replaces `path` with `circle` element to make ‘C’.
* Simplifies path for plus symbols
