Docs/add rust version requirement (#3644)

* docs: mention required Rust version in README

* docs: add rustup update instruction for minimum Rust version

* done the changes

Author: Aditya30ag

README.md

@@ -29,6 +29,13 @@ feel free to add an AST-based input for structured fuzzing, and more.
 
 - **The Rust development language**
   - We highly recommend *not* to use e.g. your Linux distribution package as this is likely outdated. So rather install Rust directly, instructions can be found [here](https://www.rust-lang.org/tools/install).
+  - The minimum supported Rust version is defined. You can always check the currently required version in LibAFL’s [Cargo.toml](https://github.com/AFLplusplus/LibAFL/blob/main/crates/libafl/Cargo.toml):
+
+    If your installed Rust version is older than the one listed in Cargo.toml, update to the latest stable toolchain:
+
+    ```bash
+    rustup update stable
+    ```
 - **LLVM tools**
   - The LLVM tools (including clang, clang++) are needed (newer than LLVM 15.0.0 up to LLVM 18.1.3) If you are using Debian/Ubuntu, again, we highly recommmend that you install the package from [here](https://apt.llvm.org/)
   - (In `libafl_concolic`, we only support LLVM version newer than 18)