docs: Add manual installation instruction for zkstack_cli (#3160)

## What ❔

Add manual installation instruction for zkstack_cli.
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## Why ❔

Using `curl | bash` could be problematic, we should provide better way.
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## Checklist

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- [x] PR title corresponds to the body of PR (we generate changelog
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- [ ] Tests for the changes have been added / updated.
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lint`.

zkstack_cli/README.md

@@ -30,6 +30,16 @@ zkstackup  --local
 
 This command installs `zkstack` from the current repository.
 
+#### Manual installation
+
+Run from the repository root:
+
+```bash
+cargo install --path zkstack_cli/crates/zkstack --force --locked
+```
+
+And make sure that `.cargo/bin` is included into `PATH`.
+
 ### Foundry Integration
 
 Foundry is used for deploying smart contracts. Pass flags for Foundry integration with the `-a` option, e.g.,