connectioln_pool: clarify how to give grants

Add a short guide how to setup Tarantool user to able to work with
connection pool.

Follows up #196

Author: DifferentialOrange

CHANGELOG.md

@@ -13,6 +13,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
   ConnectionPool is supported only for Python 3.7 or newer.
   Authenticated user must be able to call `box.info` on instances.
+  For example, to give grants to `'guest'` user, evaluate
+  ```lua
+  box.schema.func.create('box.info')
+  box.schema.user.grant('guest', 'execute', 'function', 'box.info')
+  ```
+  on Tarantool instances.
 
   ConnectionPool updates information about each server state (RO/RW)
   on initial connect and then asynchronously in separate threads.

tarantool/connection_pool.py

@@ -220,6 +220,12 @@ def __init__(self,
 
         :param list addrs: List of {host: , port:} dictionaries,
         describing server addresses.
+        :user str Username used to authenticate. User must be able
+        to call box.info function. For example, to give grants to 
+        'guest' user, evaluate
+          box.schema.func.create('box.info')
+          box.schema.user.grant('guest', 'execute', 'function', 'box.info')
+        on Tarantool instances.
         :param int reconnect_max_attempts: Max attempts to reconnect
         for each connection in the pool. Be careful with reconnect
         parameters in ConnectionPool since every status refresh is