Add documentation for LINUX_PORTABLE variable

Description

Adds a single line of documentation to README.md regarding the LINUX_PORTABLE build variable.

Motivation and Context

Package maintainers will find it useful to know how to get cmake to emit files in the correct structure for system-level packaging. I couldn't find documentation elsewhere, and it seems prudent to provide this for the example repo rather than open a PR for every OBS plugin using this template. I ran into this as an issue packaging Tuna for NixOS, but it's likely relevant for other packages as well.

How Has This Been Tested?

Documentation change, should not require testing.

Types of changes

Documentation (a change to documentation pages)

Checklist:

[ ] My code has been run through clang-format. (No code change)
[x] I have read the contributing document.
[x] My code is not on the master branch.
[ ] The code has been tested. (No code change)
[x] All commit messages are properly formatted and commits squashed where appropriate.
[x] I have included updates to all appropriate documentation.

obsproject / obs-plugintemplate