message: added note to changelog under section 0.8.1
diff: # CHANGELOG
  +## 0.8.1 (not yet released)
  +
  +**cosmwasm-vm**
  +
  +- Deprecated `Instance::get_gas` in favour of `Instance::get_gas_left`. The old
  + method will remain available for a while but will issue a deprecation warning
  + when used.
  +
  ## 0.8.0 (not yet released)
  **all**
  