add some documentation for getting started with development

This is really not hard, but even easier if one sees directly what to do.

README.md

@@ -0,0 +1,18 @@
+# Sprechstundentool der FS Mathematik TU Darmstadt
+
+## Developing
+Make sure you have [Go installed](https://go.dev/doc/install),
+e.g. by executing `go version`.
+
+Initialize the database. For developing, we recommend using sqlite:
+
+    user@host:repo/path$ sqlite3 officeHours.db -init sqlite.sql
+    sqlite> .read dummydatasqlite.sql
+
+Now start the development webserver, note that you need to manually
+restart it to code changes take effect.
+
+    go run .
+
+The website is exposed at <http://localhost:8080>, this (and some other options,
+e.g. where mails are sent to) can be adjusted in `config/config.json`.