Update build.md

[gpx1000]

Description

Add in Apple Silicon knowledge to the Readme instructions. Require minimum CMake of 3.19.2 and XCode v12

Fixes #220

General Checklist:

Please ensure the following points are checked:

• My code follows the coding style
• I have reviewed file licenses
• I have updated the header Copyright to reflect the current year (CI build will fail if Copyright is out of date)
• I have commented any added functions (in line with Doxygen)
• I have commented any code that could be hard to understand
• My changes do not add any new compiler warnings
• My changes do not add any new validation layer errors or warnings
• I have used existing framework/helper functions where possible
• My changes build on Windows, Linux, macOS and Android. Otherwise I have documented any exceptions
• My changes do not add any regressions
• I have tested every sample to ensure everything runs correctly
• This PR describes the scope and expected impact of the changes I am making

Sample Checklist

If your PR contains a new or modified sample, these further checks must be carried out in addition to the General Checklist:

• I have tested the sample on at least one compliant Vulkan implementation
• If the sample is vendor-specific, I have tagged it appropriately
• I have stated on what implementation the sample has been tested so that others can test on different implementations and platforms
• Any dependent assets have been merged and published in downstream modules
• For new samples, I have added a paragraph with a summary to the appropriate chapter in the samples readme

[CLAassistant]

All committers have signed the CLA.