notes on adding shape functions

Co-authored-by: Joshua-Kloepfer <[email protected]>

Author: cwsmith

docs/addingShapeFunctions.md

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+In order to add shape function, you must create a struct in
+https://github.com/SCOREC/meshFields/blob/main/src/MeshField_Shape.hpp.
+
+You must define numNodes, meshEntDim, and Order as const variables as well as the functions
+getValues and getLocalGradients. You then need to define a mapping from how you store the
+nodes to the ordering of the shape function defined earlier. The Omegah mappings are in
+https://github.com/SCOREC/meshFields/blob/main/src/MeshField.hpp#L63. In the Jacobian
+tests we also an identity mapping defined as another example:
+https://github.com/SCOREC/meshFields/blob/main/test/testElementJacobian2d.cpp#L12.
+
+In order to use integration with the added shape function it must be added here:
+https://github.com/SCOREC/meshFields/blob/main/src/MeshField_Integrate.hpp#L44.
+
+Finally, when trying to compute an integral using the shape function, a class derived from the
+class Integrator (defined here:
+https://github.com/SCOREC/meshFields/blob/main/src/MeshField_Integrate.hpp#L204) must be
+declared and should implement the atPoints function. An example of this can be found here:
+https://github.com/SCOREC/meshFields/blob/main/test/testCountIntegrator.cpp#L41.