docs(contracts): upgrading documenation

bolt-contracts/docs/upgrading.md

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+# Upgrading
+
+When upgrading contracts, always keep the old implementation of the contracts around and increment the version number.
+For example, when upgrading `BoltManagerV1`, copy it into a new file called `BoltManagerV2` and make your changes.
+
+This is needed to reference check the new contracts with the old contracts so that the OpenZeppelin Upgrades library can
+validate the safety of the upgrade. You MUST add this reference when upgrading a contract:
+
+```sol
+Options memory opts;
+opts.referenceContract = "BoltManagerV1.sol";
+Upgrades.upgradeProxy(proxy, "BoltManagerV2.sol", "", opts);
+```
+
+Before an upgrade, update the [`Upgrade.s.sol`](../script/holesky/Upgrade.s.sol) script to include the correct contracts, references and configurations.