mock: Update README.md with design pattern
mock: Update README.md with design pattern

Add the reasoning behind the design pattern of
placing all params in one struct.

BRANCH=none
BUG=none
TEST=Examined in Gitlies

Change-Id: I80f29468126c3a3a36363c857d52d3baad785638
Signed-off-by: Craig Hesling <383db090fd8ef907a606ac8a9eccd2962bc489cf@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/1880578
Reviewed-by: Daisuke Nojiri <fd5f93af191bf7e8f73ea71cc3c0f66b41b1dd49@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec