add software bill of materials

BILL-OF-MATERIALS.md

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+# Software Bill of Materials
+
+This is an overview of the external software components (libraries, etc.) that
+are used in fediverse.space, or that are likely to be used.
+
+## Backend
+
+I am currently in the process of migrating from a Python and Django-based
+backend to one written in Elixir. This list is what *will* be used in the near
+future.
+### Crawler and API
+* [Elixir](https://elixir-lang.org/)
+* [Phoenix](https://phoenixframework.org/)
+* [HTTPoison](https://hexdocs.pm/httpoison/readme.html)
+* See [/backend/mix.env](/backend/mix.env) for a complete overview of
+  dependencies
+
+### Graph layout
+* [Gephi toolkit](https://gephi.org/toolkit/)
+
+## Frontend
+* [React](https://reactjs.org/)
+* [Blueprint](https://blueprintjs.com/)
+* See [/frontend/package.json](/frontend/package.json) for a complete overview
+  of dependencies
+
+## Other
+* [Docker](https://www.docker.com/) and
+  [docker-compose](https://docs.docker.com/compose/overview/)
+* The frontend is hosted on [Netlify](https://www.netlify.com/)
+