Add build instructions

README.md

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+# flowsolver
+
+Solves Flow (aka Numberlink) puzzles using the Z3 constraint solver.
+
+## building
+
+Depends on Raylib and Z3.
+
+To generate build files, you can run CMake normally:
+```
+mkdir build
+cd build
+cmake .. # Add your preferred CMake options here, such as -G or -DCMAKE_BUILD_TYPE
+```
+
+And then run whatever generator you configured CMake to use.
+
+### on windows without vcpkg
+
+When building on Windows without VcPkg, you will need to point CMake to Z3, and
+create Raylib CMake files for CMake to point at. To create the Raylib files, it
+should be OK to copy the Z3 `lib/cmake/` files and modify the version code and
+the library handling code. As a general rule:
+- `z3::libz3` -> `raylib`
+- `bin/libz3.dll` -> `lib/raylib.dll`
+- `lib/libz3.lib` -> `lib/raylib.lib`
+- `MAJOR.MINOR.BUGFIX.TWEAK` (z3) -> `MAJOR.MINOR.BUGFIX` (raylib)
+
+Then, you can point CMake to these files, by setting `CMAKE_PREFIX_PATH` to
+`${z3_dir}\lib\cmake\z3;${raylib_dir}\lib\cmake\raylib`.
+
+Once Raylib ships with MSVC CMake files, this shouldn't need to be done.