add old migen tutorial

content/gateware/migen.md

@@ -50,6 +50,7 @@ You can find the Migen source <a href="http://github.com/m-labs/migen" target="_
 **Documentation**
 
 - <a href="https://m-labs.hk/migen/manual/" target="_blank" rel="noopener noreferrer">User guide
+- <a href="/docs/migen-tutorial.pdf" target="_blank" rel="noopener noreferrer">Tutorial</a> (outdated - in particular, the simulator has been rewritten and no longer uses Icarus)
 - <a href="https://www.wdj-consulting.com/blog/migen-port.html" target="_blank" rel="noopener noreferrer">Tutorial "Porting a New Board To Migen"</a> by cr1901
 - <a href="https://lab.whitequark.org/notes/2016-10-18/implementing-an-uart-in-verilog-and-migen/" target="_blank" rel="noopener noreferrer">"Implementing a UART in Verilog and Migen"</a> by whitequark
 - <a href="https://lab.whitequark.org/notes/2016-10-19/implementing-a-simple-soc-in-migen/" target="_blank" rel="noopener noreferrer">"Implementing a simple SoC in Migen"</a> by whitequark