Add multi-thread support

README.md

@@ -1,13 +1,30 @@
 # RefCount
 
 RefCount is a reference counting (thus the name) and garbage collection library
-written in standard C99.
+written in C.
+
+RefCount has support for
+ - breaking dependency chains via garbage collection
+ - Multiple distinct contexts allowing the tracking of many types of objects,
+   each with a possibly different memory allocation implementation
+ - Safely running in multi-threaded environments
 
 ### Building
 
+The only external requirement is [ht](https://git.zander.im/Zander671/ht). If
+you build using the instructions below, CMake will automatically download and
+build the dependency for you.
+
+For multi threading support, you need a C11 compiler that supports atomic types
+and has the `<threads.h>` header. Without threading support, you only need a C99
+compiler.
+
 You can build using:
 ```sh
-cmake -B build
+cmake -B build .
+# replace with above with
+# cmake -B build -DREFCOUNT_USE_THREADS=OFF
+# to build without threads (change the "OFF" to "ON" to re-enable threads)
 make -C build
 ```