[DOC] Add quick user guide to README.md.

README.md

@@ -14,6 +14,56 @@ This package is an extension of `Bifrost.jl`, a script written by Mikolaj Szydla
 ## Documentation
 The documentation is available at [https://ita-solar.github.io/BifrostTools.jl](https://ita-solar.github.io/BifrostTools.jl)
 
+## Quick user guide
+To load the package, type the following in the REPL
+
+```{julia}
+using BifrostTools
+```
+
+### Using `get_var`
+The function `get_var` is the main function for reading data from Bifrost simulations.
+It can read single or multiple snapshots, and it can read full data cubes or slices.
+It can read primary variables or auxiliary variables.
+
+The command
+
+```{julia}
+variable = get_var(expname, snap, expdir, variable)
+```
+loads the (primary or auxiliary) variable `variable` from snapshot `snap` in the simulation `expname` located in the directory `expdir`.
+
+By creating a `BifrostExperiment` object 
+
+```{julia}
+brxp = BifrostExperiment(expname, expdir)
+```
+we can access the mesh file
+
+```{julia}
+brxp.mesh
+```
+snapshot numbers
+```{julia}
+brxp.snaps
+```
+and the calling signature of `get_var` can be simplified
+
+```{julia}
+variable = get_var(brxp, snap, variable)
+```
+
+Using optional keyword-arguments in `get_var` allows us to convert units, destagger variables, rotate the grid, and read slices of the full cube.
+
+The command
+
+```{julia}
+"bx" = get_var(brxp, snap, "bx"; units="si", destagger=true)
+```
+will load the $x$-component of the magnetic field in SI units and destagger it to the cell center.
+
+See the [documentation](https://ITA-Solar.github.io/BifrostTools.jl/dev/) for further information and more elaborate example usage.
+
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