Add documentation

[mjsabby]

• Describe Frozen Objects, RO segments, etc.
• Explain how such a segment is created
• Open Issues
  • Things we can't serialize (fields of void*[])
  • Things we shouldn't serialize (IntPtr, Delegate, etc.)
  • Other complicated types (types with Finalizers, non-readonly fields)
  • Should we allow prototypes for MT Tokens (i.e. the method table token/cookie for int is always 0, string is always 1, etc.)
  • Current limitations around the fact that the segment is writable (i.e. not page protected)
  • Diagnostics support (clrmd, profiling apis, etc.)
  • Is the current plan for security good enough?
  • Should we support Frozen Objects via Stream, or arbitrary memory
  • How do really support unloadability?
  • Misc: GC Heap Hard Limit
  • Testing on ARM32, ARM64