Added maintenance guide

MAINTENANCE.md

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+# How to make a release
+
+- Update version number in Cargo.toml and commit the change (no need to push)
+- Run `make_release.ps1`
+- After CI completes, find the release on Github and write the changelog
+- Move the release from Draft to Published
+
+Note that the Github web UI will separate the release from the corresponding tag until published.
+
+# How to pull changes from polaris-web
+
+- Run `git submodule update --recursive --remote` or the `update_submodules.bat` script
+
+# How to change the database schema
+
+- Add a new folder under `migrations` following the existing pattern
+- Run `update_db_schema.bat` (all its should also work in bash if you're a Linux user)