Add a README file (#2)

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Author: aherrmann

LICENSE

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+MIT License
+
+Copyright (c) 2020 Andreas Herrmann
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

README.md

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+# todo app
+
+![](https://github.com/aherrmann/example_servant_elm_bazel/workflows/CI/badge.svg)
+
+A minimal todo app implemented in [Haskell][haskell] using [Servant][servant]
+on the backend, and in [Elm][elm] on the frontend, built with [Bazel][bazel]
+using [`rules_haskell`][rules_haskell] and [`rules_elm`][rules_elm].
+
+[haskell]: https://www.haskell.org/
+[servant]: https://hackage.haskell.org/package/servant
+[elm]: https://elm-lang.org/
+[bazel]: https://bazel.build/
+[rules_haskell]: https://haskell.build/
+[rules_elm]: https://github.com/EdSchouten/rules_elm
+
+## Requirements
+
+You will need the [dependencies of `rules_haskell`][dependencies]
+installed on your system to build.
+
+On MacOS you also need to set the following environment variable:
+
+```
+export BAZEL_USE_CPP_ONLY_TOOLCHAIN=1
+```
+
+[dependencies]: https://rules-haskell.readthedocs.io/en/latest/haskell.html#before-you-begin
+
+## Instructions
+
+Execute the following command to build and start the app:
+
+```
+$ bazel run //:backend
+```
+
+Then browse to `http://localhost:8080` to use the app.
+
+## Interactive Development
+
+You can use [`bazel-watcher`][bazel-watcher] to continuously rebuild the
+frontend whenever any of its sources change, like so.
+
+```
+$ ibazel build //frontend
+```
+
+You can use [`ghcid`][ghcid] to continuously reload the backend whenever
+any of its sources change, like so.
+
+```
+$ ghcid --command="bazel run //backend:backend@ghci" \
+    --test=Main.main \
+    --setup=":set args --assets bazel-bin/frontend/frontend.tar" \
+    --reload=bazel-bin/frontend/frontend.tar
+```
+
+This will load the Haskell code into a GHCi session and run the server in
+the interpreter. It will reload the session and restart the server
+whenever any of the Haskell source files or the frontend bundle changes.
+
+Note, if the dependency graph changes, e.g. when you edit a `BUILD.bazel`
+file, then you will have to restart `ghcid` itself.
+
+[bazel-watcher]: https://github.com/bazelbuild/bazel-watcher#readme
+[ghcid]: https://github.com/ndmitchell/ghcid#readme