Document include-path collision workaround, refs #222

[Dobiasd]

Summary

• Adds an "Avoiding include-path collisions on shared install prefixes" section to INSTALL.md.
• Explains when the problem occurs (multiple fplus copies on the include path of one build), why CMake-package consumers are unaffected, and the one-flag workaround for distro packagers / advanced users:
  -D CMAKE_INSTALL_INCLUDEDIR=include/FunctionalPlus.
• No code change. Default install layout (<prefix>/include/fplus/) is preserved, so non-CMake users (notably the frugally-deep audience) keep working without changes.

Background discussion: #222.

Test plan

• Render INSTALL.md and confirm the new section is correctly formatted and the link to #way-1-using-cmake resolves.