doc: add link from README to virtio-pmem and -mem docs

Add the two new features in the "Features and Compatibilities" section
of the main README.md.

Signed-off-by: Riccardo Mancini <mancio@amazon.com>

Author: Manciukic

README.md

@@ -114,6 +114,8 @@ The **API endpoint** can be used to:
   service is only available to the guest if this resource is configured.
 - Add a [vsock socket](docs/vsock.md) to the microVM.
 - Add a [entropy device](docs/entropy.md) to the microVM.
+- Add a [pmem device](docs/pmem.md) to the microVM.
+- Configure and manage [memory hotplugging](docs/memory-hotplug.md).
 - Start the microVM using a given kernel image, root file system, and boot
   arguments.
 - [x86_64 only] Stop the microVM.