README: split up into docs

docs/mysql-admutils-compatibility.md

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+# Compatibility mode with [mysql-admutils](https://git.pvv.ntnu.no/Projects/mysql-admutils)
+
+If you enable the feature flag `mysql-admutils-compatibility` (enabled by default for now), the output directory will contain two symlinks to the `musl` binary: `mysql-dbadm` and `mysql-useradm`. When invoked through these symlinks, the binary will react to its `argv[0]` and behave accordingly. These modes strive to behave as similar as possible to the original programs.
+
+```bash
+cargo build
+./target/debug/mysql-dbadm --help
+./target/debug/mysql-useradm --help
+```
+
+These symlinks are also included in the deb packages.
+
+### Known deviations from the original programs
+
+- Added flags for database configuration, not present in the original programs
+- `--help` output is formatted by clap in a modern style.
+- `mysql-dbadm edit-perm` uses the new implementation. Parsing the old logic was
+  too complex to be worth porting, and since the action is inherently interactive,
+  there shoulnd't have been any (or at least very few) scripts relying on the old
+  command API or behavior.
+- The new tools use the modern implementation to find it's configuration. If you compiled
+  the old programs with `--sysconfdir=<somewhere>`, you might have to provide `--config-file`
+  where the old program would just work by itself.
+- The order in which some things are validated (e.g. whether you own a user, whether the
+  contains illegal characters, whether the user does or does not exist) might be different
+  from the original program, leading to the same command giving the errors in a different order.