Update tutorial-ubuntu-16.04.md

export-dynamic pointer.

Author: ianamason

doc/tutorial-ubuntu-16.04.md

@@ -83,4 +83,8 @@ Turn the bitcode into a second executable binary. (optional -- just for fun and
 llc -filetype=obj httpd.bc
 clang httpd.o  -Wl,--export-dynamic -lpthread -lapr-1 -laprutil-1 -lpcre -o httpd_from_bc
 ```
-
+See [here](http://tldp.org/HOWTO/Program-Library-HOWTO/shared-libraries.html) for an explanation of the
+```
+-Wl,--export-dynamic
+```
+incantation.