generate XML sitemap

Added `bin/gen-sitemap.sh` to generate the XML sitemap.

NOTE: We are including in the XML sitemap, all files under
`archive/historic`, `bin`, `inc`, and well as `authors`.
NOTE: We are note sure these should be included in the sitemap.

Added `make gen_sitemap` to the top level `Makefile`.
Both `make quick_www` and `make www` perform `make gen_sitemap` last.

Run `make gen_sitemap` to perform the above.

bin/README.md

@@ -101,6 +101,16 @@ Usage:
 bin/gen-other-html.sh -v 1
 ```
 
+### [gen-sitemap.sh](gen-sitemap.sh)
+
+Generate an XML sitemap for the IOCCC web site.
+
+Usage:
+
+```sh
+bin/gen-sitemap.sh -v 1
+```
+
 
 ### [gen-top-html.sh](gen-top-html.sh)