Add documentation for pre-specified planning

[willu47]

In each year, the smif wrapper wipes the database tables clean, and writes in both past and future interventions.

To add you technologies into the future years you need to:

1 - add a pre-specified planning strategy to the model run config

strategies:
- type: pre-specified-planning
  description:future energy plan
  filename: ../energy_supply/strategies/plan.csv

2 - Put this file ../energy_supply/strategies/plan.csv in the data/energy_supply/strategies/ folder
And ensure that plan.csv looks like this:

name,build_year
intervention_name,2030

With as many rows as interventions you want to add.
Finally…

3 - Update the data/energy_supply/interventions/ *.csv files with the interventions you reference in the strategy

also strategy.csv name and intervention.csv name should match