Add test run entry to README

[DifferentialOrange]

It is not obvious how to prepare test environment at the first glance. It would be convenient to have a section in README which tells how to run tests locally since currently there is no CI test runs.

Start a Tarantool instance with
tarantool config.lua
and run tests with
go test

To test tarantool/multi, ...

or something like this. Alternatively, some simple shell script or Makefile can be added.