aco: Move README to README-ISA

The old "readme" is not really a readme but rather just
a bunch of notes with our findings about the GCN/RDNA ISA.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Tony Wasserka <tony.wasserka@gmx.de>
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4164>
diff --git a/src/amd/compiler/README.md b/src/amd/compiler/README-ISA.md
similarity index 100%
rename from src/amd/compiler/README.md
rename to src/amd/compiler/README-ISA.md