updated readme with gammas spec image

Author: shimwell

.flake8

@@ -137,3 +137,4 @@ openmc_source_plotter/_version.py
 
 # plots
 *.png
+src/openmc_source_plotter/_version.py

.gitignore

@@ -91,6 +91,31 @@ plot.show()
 
 ![openmc particle source direction plot](https://user-images.githubusercontent.com/8583900/143615706-3b3a8467-0233-42d6-a66c-d536c80a01d8.png)
 
+
+## Plot gamma spectrum of particles
+
+```python
+import openmc
+import openmc_source_plotter  # adds plot_gamma_emission plot to materials
+
+
+openmc.config["chain_file"] = "chain-endf.xml"
+
+my_material = openmc.Material()
+my_material.add_nuclide("Xe135", 1e-12)
+my_material.add_nuclide("U235", 1)
+my_material.add_nuclide("U238", 1)
+my_material.add_nuclide("Co60", 1e-9)
+my_material.volume = 1  # must be set so number of atoms can be found
+
+# adds labels to the most active 3 gamma energies
+plt = my_material.plot_gamma_emission(label_top=3)
+plt.xscale("log")  # modify axis from default settings
+plt.savefig("gamma_spec.png")
+```
+
+![openmc gamma spectrum](https://user-images.githubusercontent.com/8583900/228280129-b8160e18-9ca9-4b20-a4e1-d2948908daf6.png)
+
 ## Plot position of particles
 
 ```python
@@ -121,7 +146,9 @@ plot = my_source.plot_source_position()
 plot.show()
 ```
 
+
 ![openmc particle source position plot](https://user-images.githubusercontent.com/8583900/179424915-bee56a87-6214-46ef-8625-92b8f4cbd1b3.png)
 
 Tokamak sources can also be plotted using the [openmc-plasma-source](https://github.com/fusion-energy/openmc-plasma-source) package
+
 ![openmc_source_plotter_openmc-plasma-source_tokamak](https://user-images.githubusercontent.com/8583900/187487894-ba0bd025-46f2-4c7d-8b15-3d260aed47a0.png)

README.md

@@ -1,5 +1,4 @@
 import openmc
-import openmc.deplete
 import openmc_source_plotter  # adds plot_gamma_emission plot to materials
 
 
@@ -10,8 +9,9 @@
 my_material.add_nuclide("U235", 1)
 my_material.add_nuclide("U238", 1)
 my_material.add_nuclide("Co60", 1e-9)
-my_material.volume = 1
+my_material.volume = 1  # must be set so number of atoms can be found
 
+# adds labels to the most active 3 gamma energies
 plt = my_material.plot_gamma_emission(label_top=3)
 plt.xscale("log")  # modify axis from default settings
 plt.savefig("gamma_spec.png")