docs: add documentation for the -gc compiler flag

docs/usage.rst

@@ -129,6 +129,26 @@ There are a few flags to control how binaries are built:
     Print output of the on-chip debugger tool (like OpenOCD) while in a ``tinygo
     gdb`` session. This can be useful to diagnose connection problems.
 
+``-gc``
+    Use the specified memory manager.
+
+    ``-gc=none``
+        Do not use a memory manager at all. This will cause a link error at
+        every place in the program that tries to allocate memory. The primary
+        use case for this is finding such locations.
+
+    ``-gc=dumb``
+        Only allocate memory, never free it. This is the simplest allocator
+        possible and uses very few resources while being very portable. Also,
+        allocation is very fast. Larger programs will likely need a real garbage
+        collector.
+
+    ``-gc=marksweep``
+        Simple conservative mark/sweep garbage collector. This collector does
+        not yet work on all platforms. Also, the performance of the collector is
+        highly unpredictable as any allocation may trigger a garbage collection
+        cycle.
+
 
 Miscellaneous options
 ---------------------