cmake (Windows): recommend using Visual Studio's built-in CMake support

It is a lot more convenient to use than having to specify the
configuration in CMake manually (does not matter whether using the
command-line or CMake's GUI).

While at it, recommend using `contrib/buildsystems/out/` as build
directory also in the part that talks about running CMake manually.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

contrib/buildsystems/CMakeLists.txt

@@ -4,17 +4,25 @@
 
 #[[
 
-Instructions to run CMake:
+Instructions how to use this in Visual Studio:
 
-cmake `relative-path-to-CMakeLists.txt` -DCMAKE_BUILD_TYPE=Release
-Eg.
-From the root of git source tree
-	`cmake contrib/buildsystems/ `
-This will build the git binaries at the root
+Open the worktree as a folder. Visual Studio 2019 and later will detect
+the CMake configuration automatically and set everything up for you,
+ready to build. You can then run the tests in `t/` via a regular Git Bash.
 
-For out of source builds, say build in 'git/git-build/'
-	`mkdir git-build;cd git-build; cmake ../contrib/buildsystems/`
-This will build the git binaries in git-build directory
+Note: Visual Studio also has the option of opening `CMakeLists.txt`
+directly; Using this option, Visual Studio will not find the source code,
+though, therefore the `File>Open>Folder...` option is preferred.
+
+Instructions to run CMake manually:
+
+    mkdir -p contrib/buildsystems/out
+    cd contrib/buildsystems/out
+    cmake ../ -DCMAKE_BUILD_TYPE=Release
+
+This will build the git binaries in contrib/buildsystems/out
+directory (our top-level .gitignore file knows to ignore contents of
+this directory).
 
 Possible build configurations(-DCMAKE_BUILD_TYPE) with corresponding
 compiler flags