readme: Explain compatibility

README.md

@@ -5,9 +5,10 @@ This repository is a Linux kernel module that allows the Linux kernel to be used
 For instructions on building this module and running CCP, 
 please see our [guide](https://ccp-project.github.io/ccp-guide).
 
+[portus](https://github.com/ccp-project/portus), the CCP algorithm runtime library, in version 0.6 changed the libccp API slightly: the datapath now sends a `ready` message in `ccp_init`. This means that algorithms built on portus 0.5 or earlier won't work with the latest commits in this repository. The `portus-0.5-compat` branch should work in those cases.
 
 ## Notes
 
-- We support Linux kernel versions 4.19+ (i.e., after [our patch](https://patchwork.ozlabs.org/patch/941532/)).
-- For kernel 4.13 and 4.14, we additionally provide a compatibility mode to allow use with unmodified kernels.
+- We support Linux kernel versions 4.19+ (i.e., after [our patch](https://patchwork.ozlabs.org/patch/941532/)). The current recommended kernel version is 5.4.
+- For kernel 4.13 and 4.14, we additionally provide a compatibility mode to allow use with unmodified kernels. This compatibility mode is deprecated.
 - For other kernel versions, we suggest manually applying the patch and compiling your own kernel.