add simple description of how to pin es snapshot versions (#51225)

Author: Spencer

packages/kbn-es/README.md

@@ -59,4 +59,62 @@ Cloned location of elasticsearch repository, used when running from source
 
 Type: `String`
 
-Location where snapshots are cached
+Location where snapshots are cached
+
+## Snapshot Pinning
+
+Sometimes we need to pin snapshots for a specific version. We'd really like to get this automated, but until that is completed here are the steps to take to build, upload, and switch to pinned snapshots for a branch.
+
+To use these steps you'll need to setup the google-cloud-sdk, which can be installed on macOS with `brew cask install google-cloud-sdk`. Login with the CLI and you'll have access to the `gsutil` to do efficient/parallel uploads to GCS from the command line.
+
+ 1. Clone the elasticsearch repo somewhere
+ 2. Checkout the branch you want to build
+ 3. Run the following to delete old distributables
+
+    ```
+    find distribution/archives -type f \( -name 'elasticsearch-*-*.tar.gz' -o -name 'elasticsearch-*-*.zip' \) -not -path *no-jdk* -exec rm {} \;
+    ```
+
+ 4. Build the new artifacts
+
+    ```
+    ./gradlew -p distribution/archives assemble --parallel
+    ```
+
+ 4. Copy new artifacts to your `~/Downloads/tmp-artifacts`
+
+    ```
+    rm -rf ~/Downloads/tmp-artifacts
+    mkdir ~/Downloads/tmp-artifacts
+    find distribution/archives -type f \( -name 'elasticsearch-*-*.tar.gz' -o -name 'elasticsearch-*-*.zip' \) -not -path *no-jdk* -exec cp {} ~/Downloads/tmp-artifacts \;
+    ```
+
+ 5. Calculate shasums of the uploads
+
+    ```
+    cd ~/Downloads/tmp-artifacts
+    find * -exec bash -c "shasum -a 512 {} > {}.sha512" \;
+    ```
+
+ 6. Check that the files in `~/Downloads/tmp-artifacts` look reasonable
+ 7. Upload the files to GCS
+
+    ```
+    gsutil -m rsync . gs://kibana-ci-tmp-artifacts/
+    ```
+
+ 8. Once the artifacts are uploaded, modify `packages/kbn-es/src/custom_snapshots.js` in a PR to use a URL formatted like:
+
+    ```
+    // force use of manually created snapshots until ReindexPutMappings fix
+    if (!process.env.KBN_ES_SNAPSHOT_URL && !process.argv.some(isVersionFlag)) {
+      // return undefined;
+      return 'https://storage.googleapis.com/kibana-ci-tmp-artifacts/{name}-{version}-{os}-x86_64.{ext}';
+    }
+    ```
+
+    For 6.8, the format of the url should look like:
+
+    ```
+    'https://storage.googleapis.com/kibana-ci-tmp-artifacts/{name}-{version}.{ext}';
+    ```