Adding note to README about GCP/Azure concurrency

In reaction to:

If you do want to address the deployment race condition without using import-map-deployer, we'd love to hear what you come up with. Consider leaving a PR to these docs that explain what you did!

S3 does not allow for this strategy, but it's valid on several other storage providers as mentioned.

• Azure's docs
• Google Cloud's docs

example

To verify the process, I've implemented a basic GCP flow in a rough shell script:

#!/bin/bash
set -euxo pipefail

Bucket="${1}"
Specifier="${2}"
Destination="${3}"
JsonFile="${Bucket}/bundles/latest.json"

# Check that the target is really there
gsutil cat "${Bucket}${Destination}" | wc -c

# Operate optimistically on a single generation
# (If we race someone else with the file, only one will succeed)
CurrentVersion=$(gsutil ls -a "${JsonFile}" | cut -d'#' -f2)
gsutil cat "${JsonFile}#${CurrentVersion}" \
| jq \
    --arg specifier "$Specifier" \
    --arg target "$Destination" \
    '.imports[$specifier] = $target' \
| gsutil \
    -h "x-goog-if-generation-match: ${CurrentVersion}" \
    -h "cache-control: max-age=60" \
    cp - "${JsonFile}"

It didn't seem fit to include in the README itself, given the scope of the doc/repo.