_content/blog: add supply-chain.md
_content/blog: add supply-chain.md

Change-Id: I7e60ef59e4b425fdfcb3c9615431a426d910746d
Reviewed-on: https://go-review.googlesource.com/c/website/+/397215
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Trust: Filippo Valsorda <5116fe0442846a38fe5f19d63723d023eca87d1d@google.com>
Run-TryBot: Filippo Valsorda <5116fe0442846a38fe5f19d63723d023eca87d1d@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website