[Doc] update README.md

Author: bongjunj

README.md

@@ -6,13 +6,24 @@ Our recent development is avaiable at [our website](https://prosys.kaist.ac.kr/o
 
 ## Installation
 
+
+Configure the environment variables.
+```bash
+# The LLVM project directory.
+# We will use the clang and llvm toolchains and libraries to build
+# Optimuzz, Alive2, and other LLVM versions under test.
+export LLVM_PATH=$HOME/llvm-project
+
+# LLVM versions under test will be installed under this directory.
+export LLVM_BUILDS=$HOME/llvm-builds # or your favorite to store the LLVM project builds
+```
+
 The script below installs the followings:
 * the required OPAM packages
 * LLVM 20 with the OCaml bindings
 * Alive2
 
 ```bash
-export LLVM_PATH=$HOME/llvm-project # or your favorite
 build.sh
 ```
 
@@ -22,7 +33,6 @@ We provide following tools to employ Optimuzz.
 
 ```sh
 # Build LLVM with our instrumentation pass
-export LLVM_BUILDS=$HOME/llvm-builds # or your favorite to store the LLVM project builds
 $ tools/build.py commit <commit-sha> <target-file> [--fresh]
 $LLVM_BUILDS/llvm-builds/<commit-sha>/build/bin/opt