DOC: add internet archive link to FASTG spec

Fixes #9.

README.md

@@ -14,7 +14,9 @@ accurately representing the ambiguity resulting from sequencing limitations, plo
 or other factors that complicate representation of a seqence as a simple string.
 
 The latest specification for the FASTG format is version 1.00, as of writing;
-this specification is located [here](http://fastg.sourceforge.net/FASTG_Spec_v1.00.pdf).
+the original FASTG website is down, but
+an archived version of the FASTG v1.00 specification is accessible
+[here](https://web.archive.org/web/20211209213905/http://fastg.sourceforge.net/FASTG_Spec_v1.00.pdf).
 Whenever the rest of this documentation mentions "the FASTG spec," this is in reference
 to this version of the specification.