[motoko-san] upd README after 3rd Milestone

[GoPavel]

3rd Milestone overview:

Major changes:

• [DMS-23] Support forall and exists #25
• Better support of tuples:
  [DMS-48] motoko-san: ADT encoding for tuples #28
• Better support of more complex types (e.g. (?[Int], ?Bool):
  [DMS-52] motoko-san: Generate fields #31
• [DMS-57] [DMS-58] Support immutable records #33
• add new showcases:
  • todo_tuples.mo
    [DMS-45] add ToDO example (on tuples) #30
  • todo_records.mo

Minor changes:

• [DMS-40] Support numeric, boolean literals and wildcards in switch patterns #20
• [DMS-39] Support labels, breaks, and continues #18
• support Array.init:
  [DMS-44] motoko-san: mock the Array module #24
• [DMS-47] Support text type #27

Bugfixes:

• [DMS-12] motoko-san: Let-bound variants #21
• motoko-san: Fix a hang in reverse.mo #22
• motoko-san: Stop using (==) in trans.ml and prep.ml #23
• [DMS-51] bugfix: user postcond. should be after perms + testfix #26
• [DMS-56] Pass the correct context in dec_field' #32
• [DMS-55] Add Prim.Ret primitive that acts like var:return #34
• Improve user experience:
  [DMS-41] Fix locations for while loop invariants #29