message: Mention CG_CLIF_INCR_CACHE in the readme
diff: @@ -18,6 +18,7 @@ $ ./test.sh --release
  ### Cargo
  ```bash
  +$ export CG_CLIF_INCR_CACHE=1 # Enable caching of object files in the incremental cache
  $ CHANNEL="release" $cg_clif_dir/cargo.sh run
  ```
  @@ -26,6 +27,7 @@ If you compiled cg_clif in debug mode you should use `CHANNEL="debug"` instead o
  ### Rustc
  ```bash
  +$ export CG_CLIF_INCR_CACHE=1 # Enable caching of object files in the incremental cache
  $ rustc -Cpanic=abort -Zcodegen-backend=$cg_clif_dir/target/release/librustc_codegen_cranelift.so --sysroot $cg_clif_dir/build_sysroot/sysroot my_crate.rs
  ```
  