Update README.md

README.md

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 # aesara-theano-fallback
-Striving towards backwards compatibility with the Theano -> Aesara transition
+
+Striving towards backwards compatibility as [Theano](https://github.com/theano/theano) is replaced by
+[Aesara](https://github.com/pymc-devs/aesara) by the [PyMC3 project](https://docs.pymc.io). The idea is
+to provide a nearly drop in replacement for importing `aesara` that will fall back onto `theano` when
+`aesara` is not installed. This was specifically designed to support the [exoplanet](https://github.com/exoplanet-dev/exoplanet)
+and [starry](https://github.com/rodluger/starry) projects so it might not support all of the features
+that you need. If you find something that isn't supported, please submit a pull request!
+
+## Installaion
+
+This library can be installed using pip:
+
+```bash
+python -m pip install aesara-theano-fallback
+```
+
+## Usage
+
+The syntax is designed to mostly follow `aesara`, so things like the following will often work:
+
+```python
+import aesara_theano_fallback.tensor as aet
+```
+
+For top-level access, use
+
+```python
+from aesara_theano_fallback import aesara
+```
+
+One place where the syntax has changed significantly between Theano and Aesara is the `theano.gof`
+module was re-named to `aesara.graph` and the contents were moved around a little bit. For exoplanet
+and starry, we define a few custom `Op`s and you can use this library to do that as follows:
+
+```python
+from aesara_theano_fallback.graph import basic, op
+
+class MyPythonOp(op.Op):
+    def make_node(self, *args):
+        # ...
+        return basic.Apply(self, in_args, out_args)
+
+class MyCOp(op.ExternalCOp):
+    func_file = "./cpp_impl.cc"
+    func_name = "APPLY_SPECIFIC(my_op_name)"
+    # ...
+```