Add application schema notes to docs

[thclark]

Extracted from the old README.md of octue-app-python, and should be placed into the docs for the config schema strand:

## Application schema

For most applications, you need to supply some control parameters. For example:

 - In a simulation the user may want to specify the number of iterations or timesteps.
 - In a LiDAR analysis app, the user may want to know the deployment `lat/long` coordinates.
 - In a signal processing app, the user may want to turn on or off a particular step (e.g. smoothing of data).
 - In a Meteorological Mast analysis, the user may want to run analysis only for a specified date range.
 
It's the job of the application developer (that's you, probably) to **tell the end user what options are available**. 
The way this is done is via an **application schema**.

A **schema** describes precisely what inputs the application needs.

A **config** contains those inputs, and should match the schema.