README.msys2: Clarify the "msys2 shell" to launch
README.msys2 section "Build" says: "Launch msys2 shell." That can be awfully misleading as there really is a generic msys2 shell in the installation. One is expected to launch the build target specific shell.
Documentation updates are not discouraged, but encouraged during code freeze. Fix for this can still make it to 3.0.0-RC1.