Add README

README.md

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+# SpaceAPI Daemon
+
+`spaceapid` serves a [SpaceAPI](https://spaceapi.io)-compatible JSON on port
+8080:
+
+```shell
+curl -X GET http://localhost:8080
+```
+
+The state of the boolean `state->open` property can be modified via
+`/state/open`:
+
+```shell
+curl -X PUT -u user:password -d true http://localhost:8080/state/open
+```
+
+## Building
+
+See the `go.mod` file for minimum required Go version. There are currently no
+dependencies other than the Go standard library.
+
+```shell
+go build .
+```
+
+## Running
+
+Substitute the environment variables with appropriate custom values.
+
+```shell
+env DOORIS_USERNAME=user DOORIS_PASSWORD=password SPACE_API_JSON="$(cat ccchh-template.json)" go run .
+```