Merge pull request #1298 from aollier/gnu-rcs-link

Added link to GNU RCS

Author: ben

book/01-introduction/sections/about-version-control.asc

@@ -23,7 +23,7 @@ To deal with this issue, programmers long ago developed local VCSs that had a si
 image::images/local.png[Local version control diagram]
 
 One of the most popular VCS tools was a system called RCS, which is still distributed with many computers today.
-RCS works by keeping patch sets (that is, the differences between files) in a special format on disk; it can then re-create what any file looked like at any point in time by adding up all the patches.
+https://www.gnu.org/software/rcs/[RCS] works by keeping patch sets (that is, the differences between files) in a special format on disk; it can then re-create what any file looked like at any point in time by adding up all the patches.
 
 ==== Centralized Version Control Systems