Add Snapshot Restore README with quick-testing steps.

x-pack/plugins/snapshot_restore/README.md

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+# Snapshot Restore
+
+## Quick steps for testing
+
+### File system
+
+1. Add the file system path you want to use to elasticsearch.yml or as part of starting up ES. Note that this path should point to a directory that exists.
+
+```
+path:
+  repo: /tmp/es-backups
+```
+
+or
+
+```
+yarn es snapshot --license=trial -E path.repo=/tmp/es-backups
+```
+
+2. Use Console or UI to add a repository. Use the file system path above as the `location` setting:
+
+```
+PUT /_snapshot/my_backup
+{
+  "type": "fs",
+  "settings": {
+    "location": "/tmp/es-backups",
+    "chunk_size": "10mb"
+  }
+}
+```
+
+3. Adjust `settings` as necessary, all available settings can be found in docs:
+https://www.elastic.co/guide/en/elasticsearch/reference/master/modules-snapshots.html#_shared_file_system_repository
+
+### Readonly
+
+Readonly repositories only take `url` setting. Documentation: https://www.elastic.co/guide/en/elasticsearch/reference/master/modules-snapshots.html#_read_only_url_repository
+
+It's easy to set up a `file:` url:
+```
+PUT _snapshot/my_readonly_repository
+{
+  "type": "url",
+  "settings": {
+    "url": "file:///tmp/es-backups"
+  }
+}
+```
+
+### Source only
+
+Source only repositories are special in that they are basically a wrapper around another repository type. Documentation: https://www.elastic.co/guide/en/elasticsearch/reference/master/modules-snapshots.html#_source_only_repository
+
+This means that the settings that are available depends on the `delegate_type` parameter. For example, this source only repository delegates to `fs` (file system) type, so all file system rules and available settings apply:
+
+```
+PUT _snapshot/my_src_only_repository
+{
+  "type" : "source",
+  "settings" : {
+    "delegate_type" : "fs",
+    "location" : "/tmp/es-backups"
+  }
+}
+```
+
+### Plugin-based repositories:
+
+There are four official repository plugins available: S3, GCS, HDFS, Azure. Available plugin repository settings can be found in the docs: https://www.elastic.co/guide/en/elasticsearch/plugins/master/repository.html.
+
+To run ES with plugins:
+
+1. Run `yarn es snapshot` from the Kibana directory like normal, then exit out of process.
+2. `cd .es/8.0.0`
+3. `bin/elasticsearch-plugin install https://snapshots.elastic.co/downloads/elasticsearch-plugins/repository-s3/repository-s3-8.0.0-SNAPSHOT.zip`
+4. Repeat step 3 for additional plugins, replacing occurrences of `repository-s3` with the plugin you want to install.
+5. Run `bin/elasticsearch` from the `.es/8.0.0` directory. Otherwise, starting ES with `yarn es snapshot` would overwrite the plugins you just installed.