Update README

Add possibility of using DEFAULT as target. Explaining how and current
operators situation.

README.md

@@ -416,6 +416,14 @@ advisable for NVIDIA and **mandatory for AMD** to provide the specific device
 architecture through `-DDPCPP_SYCL_ARCH=<arch>`, e.g., `<arch>` can be `sm_80`
 for NVIDIA or `gfx908` for AMD.
 
+It is possible to use the `DEFAULT` target even for AMD and NVIDIA GPUs, but
+defining `-DDPCPP_SYCL_TARGET` and `-DDPCPP_SYCL_ARCH` is mandatory. The rules
+mentioned above also apply in this case.
+Using `DEFAULT` as the target will speedup compilation time at expense of
+runtime. Moreover, currently, some operators don't work, and therefore, their
+tests are disabled. For full compatibility and performance, set also `TUNING_TARGET`
+to the appropriate one.
+
 #### DPC++ Compiler Support
 
 As DPCPP SYCL compiler the project is fully compatible with `icpx` provided by