add the example to doc

doc/init/init-reaction.md

@@ -9,6 +9,13 @@ To start the workflow, one needs a box containing reactive systems. The followin
 
 The Exploring step uses LAMMPS [pair_style reaxff](https://docs.lammps.org/latest/pair_reaxff.html) to run a short ReaxMD NVT MD simulation. In the Sampling step, molecular clusters are taken and k-means clustering algorithm is applied to remove the redundancy, which is described in [Nature Communications, 11, 5713 (2020)](https://doi.org/10.1038/s41467-020-19497-z). The Labeling step calculates energies and forces using the Gaussian package.
 
+An example of `reaction.json` is given below:
+
+```{literalinclude} ../../examples/init/reaction.json
+:language: json
+:linenos:
+```
+
 For detailed parameters, see [parametes](init-reaction-jdata.rst) and [machine parameters](init-reaction-mdata.rst).
 
 The genereated data can be used to continue DP-GEN concurrent learning workflow. Read [Energy & Fuels, 2021, 35 (1), 762–769](https://10.1021/acs.energyfuels.0c03211) for details.