docs: add installation instructions without cloning (#213)

**Motivation**

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**Description**

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<!-- Link to issues: Resolves #111, Resolves #222 -->

Signed-off-by: Manuel Iñaki Bilbao <manuel.bilbao@lambdaclass.com>

Author: ManuelBilbao

README.md

@@ -27,6 +27,12 @@ To install it, you need to clone the repository and run the following command to
 make cli
 ```
 
+Or install without cloning:
+
+```shell
+cargo install --git https://github.com/lambdaclass/rex --locked
+```
+
 ### Using the CLI
 
 After installing the CLI with `make cli`, run `rex` to display the help message and see the available commands.