Update README

Issue/feature addressed

Due to the large number of back-ends available (Serial, AVX2/AVX512, CUDA, SYCL, Kokkos), compilation of the redesign can be difficult for inexperienced users. This MR proposes to add a minimalistic compilation command example for each available back-end.

Proposed solution

Add minimalist cmake compilation instruction for the following cases:

• Serial
• AVX2
• AVX512
• CUDA
• SYCL (Serial)
• SYCL (CUDA)
• Kokkos (Serial)
• Kokkos (CUDA)

Implementation

Tests

Suggested reviewers

Please suggest any people who would be appropriate to review your code.

Notes

Please add any other information that could be useful for reviewers.

Checklist

• [ ] Functions and classes, or changes to them, are documented.
• [ ] User guide/documentation is updated.
• [ ] Changelog is updated.
• [ ] Suitable tests added for new functionality.
• [ ] Contributed code is correctly formatted. (See the contributing guidelines).
• [ ] License added to any new files.
• [ ] No extraneous files have been added (e.g. compiler output or test data files).