Clarify that buck2 version is important (#734)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/734

The buck2 commandline tool version must be compatible with the buck2 prelude version. Make it clear that users must grab a specific version of buck2.

Reviewed By: larryliu0820

Differential Revision: D50093420

fbshipit-source-id: e7769557791e200066e033d3c443e3f27db83354