ipxe: Add documentation

Signed-off-by: Michael Brown <[email protected]>

.github/README.md

@@ -0,0 +1,26 @@
+# shim for iPXE
+
+This is a lightly modified version of the general-purpose [shim][shim]
+used to enable UEFI Secure Boot for open source projects.
+
+This fork includes modifications to simplify the use of shim with
+[iPXE][ipxe].  In particular, this shim is built to trust the iPXE
+project's [EV code signing certificate](../ipxe.der), and includes
+logic to automatically determine the iPXE filename based on the name
+used for the shim itself, by stripping out the shim portion of the
+filename.  For example:
+
+| shim filename         | iPXE filename         |
+| :-------------------- | :-------------------- |
+| ipxe-shimx64.efi      | ipxe.efi              |
+| intel-shimx64.efi     | intel.efi             |
+| snponly-shimx64.efi   | snponly.efi           |
+
+The UEFI Secure Boot signed binaries can be downloaded from the
+releases listed at https://github.com/ipxe/shim/releases
+
+(Note that only full releases are signed: prereleases will contain
+unsigned binaries.)
+
+[shim]: https://github.com/rhboot/shim
+[ipxe]: https://ipxe.org