Fix footnotes markdown syntax in `README.md`.
@@ -98,7 +98,7 @@ TensorFlow Federated | TensorFl
[0.10.1](https://github.com/tensorflow/federated/tree/v0.10.1) | [tensorflow 2.0.0](https://pypi.org/project/tensorflow/2.0.0/)
[0.10.0](https://github.com/tensorflow/federated/tree/v0.10.0) | [tensorflow 2.0.0](https://pypi.org/project/tensorflow/2.0.0/)
[0.9.0](https://github.com/tensorflow/federated/tree/v0.9.0) | [tf-nightly 2.1.0.dev20191005](https://pypi.org/project/tf-nightly/2.1.0.dev20191005/)
-~~[0.8.0](https://github.com/tensorflow/federated/tree/v0.8.0)~~ [^1] | [tf-nightly 1.15.0.dev20190805](https://pypi.org/project/tf-nightly/1.15.0.dev20190805/)
+~~[0.8.0](https://github.com/tensorflow/federated/tree/v0.8.0)~~ <sup>[1](#footnote1)</sup> | [tf-nightly 1.15.0.dev20190805](https://pypi.org/project/tf-nightly/1.15.0.dev20190805/)
[0.7.0](https://github.com/tensorflow/federated/tree/v0.7.0) | [tf-nightly 1.15.0.dev20190711](https://pypi.org/project/tf-nightly/1.15.0.dev20190711/)
[0.6.0](https://github.com/tensorflow/federated/tree/v0.6.0) | [tf-nightly 1.15.0.dev20190626](https://pypi.org/project/tf-nightly/1.15.0.dev20190626/)
[0.5.0](https://github.com/tensorflow/federated/tree/v0.5.0) | [tf-nightly 1.14.1.dev20190528](https://pypi.org/project/tf-nightly/1.14.1.dev20190528/)
@@ -107,7 +107,7 @@ TensorFlow Federated | TensorFl
[0.2.0](https://github.com/tensorflow/federated/tree/v0.2.0) | [tensorflow 1.13.1](https://pypi.org/project/tensorflow/1.13.1)
[0.1.0](https://github.com/tensorflow/federated/tree/v0.1.0) | [tensorflow 1.13.0rc2](https://pypi.org/project/tensorflow/1.13.0rc0/)
-[^1]: TensorFlow Federated
+<sup id="footnote1">1</sup> TensorFlow Federated
[0.8.0](https://github.com/tensorflow/federated/tree/v0.8.0) fails to pip
install `pip install tensorflow_federated==0.8.0` because it requires
`tf-nightly==1.15.0.dev20190805`, which is no longer available.
