Updated documentation.md to include input for stars.

#### The staging configuration file (`etl/stg/config/config.yml`)

The staging config file has two main categories:

The staging config file has three main categories:

* **grid:**

    * grid_type: regular or spherical

    * velocity_gradient: the value of the velocity gradient for solid-body roation.

    * velocity_gradient_unit: the unit of the velocity gradient, e.g. "km/s/pc"

* **stars:**

  Adding the star configuration section makes the program compute the dust temperature distribution given the properties

  of the stars included. _Caveat:_ be careful, test runs show unexpectedtly low dust temperatures using a blackbody.

    * nstars: the number of stars to include

    * rstars: the radius of the star in cm; it is normally ignored, unless the parameter `istar_sphere` is set to 1

    * mstars: the mass in grams; not used in the current version of RADMC

    * star_positions: x, y, z coordinates of the star in cm, expressed as a list of lists

    * star_fluxes: stellar fluxes at each of the computed lambdas in erg cm$^-2$ s$-1$ Hz$-1$; if negative, interpreted

      as the peak temperature of a blackbody

    * nlambdas: number of wavelengths to compute

    * spacing: log or linear

    * lambdas_micron_limits: the limits in wavelength to consider in the run, expressed in micron

* **lines:**

    * species_to_include: the list of molecular species to include in the RADMC postprocessing, e.g. ['e-ch3oh']

    * molecular_abundances: a dict-like parameter, containing the species name and the corresponding fractional