INSTALL-CMAKE.md: explain cmake -G <generator-name>

- Explain that CMake's -G option can be used to specify which build
  system to generate files for.

Example: cmake ../curl -G "MinGW Makefiles"

Ref: curl#12224 (comment)

Closes curl#13244

docs/INSTALL-CMAKE.md

@@ -66,6 +66,13 @@ If you want to build in the source tree, it is enough to do this:
 
     $ cmake .
 
+### Build system generator selection
+
+You can override CMake's default by using `-G <generator-name>`. For example
+on Windows with multiple build systems if you have MinGW-w64 then you could use
+`-G "MinGW Makefiles"`.
+[List of generator names](https://cmake.org/cmake/help/latest/manual/cmake-generators.7.html).
+
 ## Using `ccmake`
 
 CMake comes with a curses based interface called `ccmake`. To run `ccmake`