Update README.md

Added information about necessary "make install" step. From my experience, on Win 11 without having the lib installed and its target location referred from the custom project (C:/Program Files (x86)/mega65libc) cmake configuration would fail.

README.md

@@ -36,9 +36,12 @@ Simple C library for the MEGA65
    cd mega65-libc
    cmake -DCMAKE_PREFIX_PATH=$HOME/llvm-mos -B build
    make
+   make install # install library files
    make test # if `xmega65` (Xemu) was in your path when running cmake
    ~~~
 
+Installed artifacts should be made visible to cmake's package search, e.g. by adding their path to CMAKE_PREFIX_PATH.
+
 #### Dependent projects
 
 - Classic `Makefile` projects should use `CC=mos-mega65-clang`.