Add documentation

Author: leonardehrenfried

docs/Migrating-Configuration.md

@@ -0,0 +1,27 @@
+While OTP's GraphQL APIs are very, very stable even across versions, the JSON configuration schema
+is not. Changes to it are relatively frequent and you can see all PRs that change it with
+the [Github tag 'config change'](https://github.com/opentripplanner/OpenTripPlanner/pulls?q=label%3A%22config+change%22).
+
+### Migrating the JSON configuration
+
+OTP validates the configuration and prints warnings during startup. This means that when you
+migrate to a newer version, you should carefully inspect the logs. If you see messages like
+
+```
+(NodeAdapter.java:170) Unexpected config parameter: 'routingDefaults.stairsReluctance:1.65' in 'router-config.json'
+```
+
+this means there are properties in your configuration that are unknown to OTP and therefore likely
+to be a configuration error, perhaps because the schema was changed. In such a case you should
+consult the [feature](Configuration.md#otp-features), [router](RouterConfiguration.md) or 
+[build configuration documentation](BuildConfiguration.md) to find out what he new schema looks like.
+
+### Aborting on invalid configuration
+
+If you want OTP to abort the startup when encountering invalid configuration, you can add the flag
+`--configCheck` to your regular OTP CLI commands.
+
+This should of course be used wisely as it can also accidentally make your production instances refuse 
+to start up.
+Therefore, we recommend that you use it only in a separate pre-production step, perhaps during graph
+build.

mkdocs.yml

@@ -75,6 +75,7 @@ nav:
     - Router: 'RouterConfiguration.md'
     - "Route Request": 'RouteRequest.md'
     - "Realtime Updaters": 'UpdaterConfig.md'
+    - "Migrating Configuration": 'Migrating-Configuration.md'
 - Features explained:
     - "Routing modes": 'RoutingModes.md'
     - "In-station navigation": 'In-Station-Navigation.md'