Document a bit gae.

appengine/README.md

 # appengine/
 
 This directory contains the AppEngine services needed for a LUCI infrastructure.
 
 Every single microservice below is _optional_. Please mix and match based on
 your needs.
 
 
 ## TL;DR
 
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     third parties that are not meant to be ever used on a AppEngine server.
 
 
 ## Tooling
 
 All services can be managed with `./tools/gae`, including running locally or
 pushing a new version. Use `./tools/gae help` for an up to date list of commands
 available.
 
 
+### Pushing
+
+Pushing new code to an AppEngine instance doesn't change the default version.
+
+To push a new version of one of the services, do:
+
+```
+cd <server> # for example, config_service
+./tools/gae upload -A <instance_name>
+```
+
+As described in the output by the tool, you can access it to
+`<version>-dot-<name>.appspot.com` until you switch the default version.
+
+
+### Changing the version
+
+To make the new code _live_, you need to change the default version:
+
+```
+cd <server> # for example, config_service
+./tools/gae switch -A <instance_name>
+```
+
+`gae` will propose the versions already uploaded and will propose the latest one
+by default.
+
+
 ## External dependencies
 
 luci-py leverages Chromium specific functionalities:
 [CIPD](https://chromium.googlesource.com/infra/infra/+/master/cipd/)
 (hermetic package management) and
 [ts_mon](https://chromium.googlesource.com/infra/infra/+/master/infra_libs/ts_mon/)
 (monitoring).  Neither are strictly required for operational purpose.