Add explanation for clone use

    return (*reinterpret_cast<std::function<int()>*>(arg))();

  };

  // See README.md for why we create the child process this way

  child_pid_ = clone(proxy, stack_->top(), CLONE_VM | CLONE_FS | CLONE_FILES /*|CLONE_UNTRACED*/,

                     reinterpret_cast<void*>(&func_));

  if (child_pid_ < 0) {

 1. *Original process*: Leak detection is requested by calling `GetUnreachableMemory()`

 2. Allocations are disabled using `malloc_disable()`

 3. The collection process is spawned.  The collection process is similar to a normal `fork()` child process, except that it shares the address space of the parent - any writes by the original process are visible to the collection process, and vice-versa.

 3. The collection process is spawned.  The collection process, created using clone, is similar to a normal `fork()` child process, except that it shares the address space of the parent - any writes by the original process are visible to the collection process, and vice-versa. If we forked instead of using clone, the address space might get out of sync with observed post-ptrace thread state, since it takes some time to pause the parent.

 4. *Collection process*: All threads in the original process are paused with `ptrace()`.

 5. Registers contents, active stack areas, and memory mapping information are collected.

 6. *Original process*: Allocations are re-enabled using `malloc_enable()`, but all threads are still paused with `ptrace()`.