Add README.md for adding user extensions

Author: rpep

fidimag/user/README.md

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+# User Extensions
+
+The user extensions directory is here to allow for 
+the user to be able to straightforwardly run compiled
+code within Fidimag for performance reasons. We
+consider this an advanced feature and do not recommend
+trying this unless you have experience writing and
+building C/Cython programs.
+
+Some of the energy classes perform callbacks to 
+user-supplied functions. Performance for this 
+is generally poor, as there is an overhead to 
+calling Python functions repeatedly. Hence,
+we place this folder here to allow you to expose
+functions written in Cython/C conveniently.
+
+An example has been supplied. We suggest copying
+the folder and modifying each of the files in 
+this. Please note that we have automated the
+building of the extensions, but you can only
+have a single Cython .pyx file per directory, 
+because a single Cython module is created in
+each folder. The module that is created will have 
+the name of this file.
+
+You do not explicitly need to write an __init__.py file;
+your extension will be importable immediately from
+fidimag.extensions.user.$FOLDERNAME
+
+The __init__.py file lets you do the slightly shorter:
+from fidimag.user.$FOLDERNAME import *
+