Add migration guide.

docs/migration-from-mxisd.md

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+# Migration from mxisd
+
+Version 2.0.0 of the ma1sd uses the same format of the database schema and main configuration file as mxisd.
+
+Migration from mxisd:
+- install ma1sd via deb package, docker image or zip/tar archive
+- stop mxisd
+- copy configuration file (by default /etc/mxisd/mxisd.yaml to /etc/ma1sd/ma1sd.yaml)
+- copy key store (by default /var/lib/mxisd/keys folder to /var/lib/ma1sd/keys)
+- copy storage (by default /var/lib/mxisd/store.db to /var/lib/ma1sd/store.db)
+- change paths in the new config file (ma1sd.yaml). There are options: `key.path` and `storage.provider.sqlite`
+- start ma1sd
+
+Due to ma1sd uses the same ports by default as mxisd it isn't necessary to change nginx/apache configuration.
+
+If you have any troubles with migration don't hesitate to ask questions in [#ma1sd:ru-matrix.org](https://matrix.to/#/#ma1sd:ru-matrix.org) room.