rar: added rar package and algorithm

wiki/rar_(algorithm).md

@@ -6,6 +6,9 @@ For the linux `rar` or `unrar` package see the
 
 RAR - short for Roshal Archive - is an algorithm for compression aswell as a
 format (`.rar`).
-RAR archives can feature files with extensions of form `.r01`, `.r02`, etc.
+RAR archives can feature files with extensions of an `r` with two digits
+following like `.r00` or `.r01`.
+When more than 100 data files are needed the letter will turn to an `s` and so
+forth until enough files have been created.
 These are parts of the compressed data that are created when the data is
 compressed.