Apply prjxray ideas to document the bitstream for Spartan 6 parts

# [prjxray] Apply prjxray ideas to document the bitstream for Spartan 6 parts

# Brief explanation

Spartan 6 is a hugely popular part which would be awesome to have support for in SymbiFlow

## Expected results

* Documentation of the bitstream for Spartan 6 parts.
* Support for using Spartan 6 in the SymbiFlow (IE Verilog to Routing).

# Detailed Explanation

SymbiFlow currently only supports Xilinx Series 7 parts and the Lattice iCE40 parts. While the Spartan 6 has mostly be superseded by the Artix 7 and Spartan 7 there are still a **huge** number of boards out there with Spartan 6 parts. Due to its huge popularity it will be a long time until the part is no longer in use (people still start new designs with Spartan 3!).

To make it even more important Spartan 6 designs you still have to use ISE which is significantly worse then Vivado in many ways. Having an non-ISE toolchain for that would be **awesome**.

Spartan 6 is used heavily by a number of open source projects;
* The Novena Open Laptop - https://www.crowdsupply.com/sutajio-kosagi/novena
* TimVideos HDMI2USB Project - https://hdmi2usb.tv

## Further reading

* [Link to a PDF document](http://abc/abc.pdf)
* [Link to a some HTML page](http://abc/abc.html)

# Knowledge Prerequisites

* List of
* what knowledge is required
* to complete this project.