Mojo: add more contents to README.md.

This CL:
* Describes conversion rules between different versions of structs.
* Adds general guidelines to ensure backward-compabile behavior.
* Clarifies usage of nullable/non-nullable primitive numeric types.

BUG=None

Change-Id: Ia61f4c1135a397722422999bd30dc1d915a71c8c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5431464
Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Yuzhu Shen <yzshen@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1286126}