Added a quick 'documentation.md' file, to describe where the Lighthouse technical documentation is, and how it can be updated.

docs/documentation.md

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+# Lighthouse Technical Documentation
+
+The technical documentation, as generated by Rust, is available at [lighthouse-docs.sigmaprime.io](http://lighthouse-docs.sigmaprime.io/).
+
+This documentation is generated from Lighthouse and updated regularly.
+
+
+### How to update:
+
+- `cargo doc`: builds the docs inside the `target/doc/` directory.
+- `aws s3 sync target/doc/ s3://lighthouse-docs.sigmaprime.io/`: Uploads all of the docs, as generated with `cargo doc`, to the S3 bucket.
+
+**Note**: You will need appropriate credentials to make the upload.
+