document cmake 3.22.1 requirement

Signed-off-by: Steve Peters <[email protected]>

README.md

@@ -174,6 +174,8 @@ sdformat supported cmake parameters at configuring time:
 If you want to build Python bindings separately from the main libsdformat library
 (for example if you want to build Python bindings for multiple versions of Python),
 you can invoke cmake on the `python` folder instead of the root folder.
+This requires cmake version 3.22.1 due to the use of a `CMAKE_REQUIRE_FIND_PACKAGE_*`
+variable, which is newer than the minimum required version of cmake for sdformat14.
 Specify the path to the python executable with which you wish to build bindings
 in the `Python3_EXECUTABLE` cmake variable.
 Specify the install path for the bindings in the `CMAKE_INSTALL_PREFIX`