+add docs

Signed-off-by: lizzie <lizzie@eden-emu.dev>

src/dynarmic/docs/PowerPC.md

@@ -0,0 +1,6 @@
+# PowerPC 64 backend
+
+The ppc64 backend currently only supports the little endian variant, with big endian support being experimental for the time being. Additionally only A32 is supported (for now).
+
+- Flag handling: Flags are emulated via software, while there may be some funny tricks with the CTR, I'd rather not bother - plus it's widely known that those instructions are not nice on real metal - so I would rather take the i-cache cost.
+- 128-bit atomics: No 128-bit atomic support is provided, this may cause wrong or erroneous execution in some contexts.