[SYCL][Doc] Linked extension process readme from dpcpp readme. (#12332)

This small addition signposts interested users to instructions on adding
DPC++ extensions. It was motivated by
#12251 (comment)
When looking into the logic of the docs, I found there was no
signposting from the initial repo readme to tell contributors how to
propose their own extensions.
There have been some important third-party contributions to dpc++ and in
the future it is possible such contributions could include proposing new
oneapi extensions.

Signed-off-by: JackAKirk <[email protected]>

Author: JackAKirk

sycl/doc/developer/ContributeToDPCPP.md

@@ -10,6 +10,8 @@ All changes made to the DPC++ compiler and runtime library should generally
 preserve existing ABI/API and contributors should avoid making incompatible
 changes. One of the exceptions is experimental APIs, clearly marked so by
 namespace or related specification.
+If you wish to propose a new experimental DPC++ extension then read
+[README-process.md](https://github.com/intel/llvm/blob/sycl/sycl/doc/extensions/README-process.md).
 
 Another exceptional case is the transition from SYCL 1.2.1 to SYCL 2020
 standard.