Warning about unhandled CMakeLists.txt file during build

[d-ronnqvist]

Building main Swift-Markdown results in the following warning

warning: 'swift-markdown': found 1 file(s) which are unhandled; explicitly declare them as resources or exclude from the target
/path/to/swift-markdown/Sources/Markdown/CMakeLists.txt

This is caused by #141

Steps to reproduce

Run swift build on the main branch (a680aed8 or later)