Added intro to grav harmonics

docs/user-guide/gravitational-harmonics/index.rst

@@ -3,6 +3,15 @@ Gravitational Harmonics
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 Here, we show how to use Swiftest's Gravitational Harmonics capabilities. 
+Swiftest uses `SHTOOLS <https://shtools.github.io/SHTOOLS/>` to compute the gravitational harmonics coefficients for a given body and calculate it's associated acceleration kick. 
+The coefficients can be computed in a number of ways: 
+- Using the axes measurements of the body ( :func: `clm_from_ellipsoid <swiftest.shgrav.clm_from_ellipsoid>`)
+- Using a surface relief grid ( :func: `clm_from_relief <swiftest.shgrav.clm_from_relief>`). 
+
+   * This function is still in development and may not work as expected.
+
+- Manually entering the coefficients when adding the central body ( :func: `add_body <swiftest.Simulation.add_body>`)
+
 This is based on ``/spherical_harmonics_cb`` in ``swiftest/examples``.