add some docs for agp_gart and intel_extreme drivers

Not a lot of info there yet, but it's a start.

docs/develop/busses/agp_gart/ReadMe.md

@@ -0,0 +1,22 @@
+AGP (and PCI-express) Graphics Address Re-Mapping Table
+=======================================================
+
+The GART is an IO-MMU allowing the videocard and CPU to share some memory.
+Either the CPU can access the video RAM directly ("aperture"), or the video
+card can access the system RAM using DMA access.
+
+The GART converts between physical addresses and virtual addresses on the
+video card side. Of course, the CPU must then map these physical addresses
+in its own address space to use them (using the MMU).
+
+The GART works as you'd expect from an MMU. It has a page table (called GTT)
+in RAM and walks it to figure out mappings. Since there cannot be page misses
+(that would require exception handling on the GPU side), access to missing
+pages are instead sent to a dedicated "scratch" page which is not used for
+anything else.
+
+Our driver implements the GART and GTT for Intel graphics card only, so far.
+Since our videodrivers are only doing modesetting, they do not need much
+support and other drivers implemented GTT management directly on their own
+(it is usually enough to make the framebuffer accessible to the CPU). However,
+this could be generalized into a more flexible iommu bus protocol.