Update Foundry docs (#1105)

docs/modules/ROOT/pages/foundry/pages/foundry-upgrades.adoc

@@ -49,6 +49,34 @@ Set the following in `remappings.txt`:
 
 NOTE: Use `LegacyUpgrades.sol` instead of `Upgrades.sol` to upgrade existing deployments that were created with OpenZeppelin Contracts v4.
 
+=== Optional: Alternative installation methods
+
+==== NPM
+
+Follow the steps above, but instead of running `forge install OpenZeppelin/openzeppelin-foundry-upgrades`, use this command instead:
+[source,console]
+----
+npm install @openzeppelin/foundry-upgrades
+----
+
+Then add the following additional lines to `remappings.txt`, in addition to the ones described above:
+[source,console]
+----
+openzeppelin-foundry-upgrades/=node_modules/@openzeppelin/foundry-upgrades/src/
+solidity-stringutils/=node_modules/@openzeppelin/foundry-upgrades/lib/solidity-stringutils/
+----
+
+==== Soldeer
+
+Follow the steps above, but instead of running `forge install OpenZeppelin/openzeppelin-foundry-upgrades`, use one of the install commands described in https://soldeer.xyz/project/openzeppelin-foundry-upgrades
+
+Then add the following additional lines to `remappings.txt`, in addition to the ones described above (replace `0.3.6` with the version of the plugin that you installed):
+[source,console]
+----
+openzeppelin-foundry-upgrades/=dependencies/openzeppelin-foundry-upgrades-0.3.6/src/
+solidity-stringutils/=dependencies/openzeppelin-foundry-upgrades-0.3.6/lib/solidity-stringutils/
+----
+
 == Foundry Requirements
 
 This library requires https://github.com/foundry-rs/forge-std[forge-std] version 1.8.0 or higher.