message: Document new requirement on allocate
diff: **cosmwasm-vm**
  +- The export `allocate` must not return 0 as a valid address. The contract is
  + responsible for avoiding this offset in the linear memory.
  - The import `db_read` now allocates memory for the return value as part of the
  call and returns a pointer to the value as `u32`. The return value 0 means
  _key does not exist_.
  