Explain app config / host config and sgx-lkl-cfg

[letmaik]

From https://github.com/lsds/sgx-lkl/pull/333/files/bd1b516276b26f97961ac932a6796ce7d0efca6d#diff-ecf6ebea1088ef4b85771da37e9edeeb:

The filesystem image contains only the files of the Docker image, but not its runtime metadata (like entrypoint, working directory, and environment variables). To run SGX-LKL we need to create configuration files containing these information. Fortunately, during conversion, this runtime metadata was queried from Docker and stored in special files named pythonapp.img.*. The sgx-lkl-cfg tool can use these files to generate an initial set of SGX-LKL host and application configuration files:

@prp wrote:

I would put it in the main README.md file, as this is fundamental to running applications with SGX-LKL. This should also include a more detailed explanation of what the host and app configs are and why both are needed.