[README] Mention make dev-install-llvm for custom LLVM build (#7279)

The make target `dev-install-llvm` was added in #6709 to streamline the
process of building custom LLVM and installing Triton using it.

While I was building Triton, I encountered the glibc version mismatch
issue (e.g., #7088, #6747, #3397), and I found the script super useful
to build LLVM from source, so I suppose it would be useful to mention it
on README.

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Author: ShawnZhong

README.md

@@ -55,6 +55,17 @@ downloads a prebuilt LLVM, but you can also build LLVM from source and use that.
 LLVM does not have a stable API, so the Triton build will not work at an
 arbitrary LLVM version.
 
+For convenience, use the following command to build LLVM and install Triton with the custom LLVM:
+
+```shell
+make dev-install-llvm
+```
+
+<details>
+<summary>
+Alternatively, follow these steps to build LLVM from source manually.
+</summary>
+
 1. Find the version of LLVM that Triton builds against.  Check
 `cmake/llvm-hash.txt` to see the current version. For example, if it says:
        49af6502c6dcb4a7f7520178bd14df396f78240c
@@ -86,6 +97,8 @@ arbitrary LLVM version.
          LLVM_SYSPATH=$LLVM_BUILD_DIR \
          pip install -e .
 
+</details>
+
 # Tips for building
 
 - Set `TRITON_BUILD_WITH_CLANG_LLD=true` as an environment variable to use clang