README: Add compile_commands.json chapter

Author: guruz

README.md

@@ -72,6 +72,20 @@ cmake . -DLLVM_CONFIG_EXECUTABLE=/usr/local/Cellar/llvm/<your_llvm_version>/bin/
 make
 ```
 
+compile_commands.json
+=====================
+We need a compile_commands.json file to know how your project is usually compiled.
+There are multiple ways to get this file:
+
+* For cmake, pass -DCMAKE_EXPORT_COMPILE_COMMANDS=ON as a cmake parameter
+* For qmake, configure/autoconf and others, follow the instructions in scripts/fake_compiler.sh or scripts/woboq_cc.js.
+These are fake compilers that append the compiler invocation to the json file and forward to the real compiler.
+Your real compiler is overriden using the CC/CXX environment variables
+Make sure to have the json file properly terminated.
+* There is also a project called Build EAR (Bear) that achieves a similar thing as our fake compilers
+but is using LD_PRELOAD to inject itself into the build process to catch how the compiler is invoked.
+https://github.com/rizsotto/Bear
+
 
 Using the generator
 ===================

scripts/fake_compiler.sh

@@ -2,15 +2,15 @@
 
 #
 # This script can be used to generate the compile_commands.json file.
-# Configure this script as the compiler.
+# Configure this script as the compiler, e.g. with the CC/CXX environment variables.
 # Export the $COMPILATION_COMMANDS environement variable to the full path of the compile_commands.json file.
 # set $FORWARD_COMPILER to the path of the actual compiler to perform the actual compilation.
 #
-# Example:
+# Example using configure (similar when using qmake):
 #
 # export COMPILATION_COMMANDS=/path/to/compile_commands.json
 # export FORWARD_COMPILER=g++
-# CXX=/path/to/fake_compiler.sh ./configure
+# CC=/path/to/fake_compiler.sh CXX=/path/to/fake_compiler.sh ./configure
 # echo "[" > $COMPILATION_COMMANDS
 # make -j1
 # echo " { \"directory\": \".\", \"command\": \"true\", \"file\": \"/dev/null\" } ]" >> $COMPILATION_COMMANDS