Add some documentation about using alloc bit (#111)

Author: qinsoon

README.md

@@ -93,6 +93,14 @@ building OpenJDK, we can change its location to be in the object's header:
 $ MARK_IN_HEADER=1 make CONF=linux-x86_64-normal-server-$DEBUG_LEVEL THIRD_PARTY_HEAP=$PWD/../../openjdk
 ```
 
+### Alloc bit
+To support the `global_alloc_bit` feature in mmtk-core, you can set the environment variable `GLOBAL_ALLOC_BIT=1` when
+building OpenJDK. This will set the feature for mmtk-core, as well as compiling the fastpath for the alloc bit.
+
+```console
+$ GLOBAL_ALLOC_BIT=1 make CONF=linux-x86_64-normal-server-$DEBUG_LEVEL THIRD_PARTY_HEAP=$PWD/../../openjdk
+```
+
 ## Test
 
 ### Run HelloWorld (without MMTk)

mmtk/Cargo.toml

@@ -29,6 +29,8 @@ default = []
 mmtk_extreme_assertions = ["mmtk/extreme_assertions"]
 nogc_lock_free = ["mmtk/nogc_lock_free"]
 nogc_no_zeroing = ["mmtk/nogc_no_zeroing"]
+# Use the env var GLOBAL_ALLOC_BIT=1 when building OpenJDK so the fastpath for setting the alloc bit will be compiled in.
+# See README.
 global_alloc_bit = ["mmtk/global_alloc_bit"]
 
 # This compile time constant places the mark bit in the header of the object instead of on the side.