Fix minor grammar and wording issues in README

[yazdanbhd]

Description

While reviewing the README, noticed a few minor grammar and wording issues that could be improved for clarity. These appear to be small typos or missing words and do not affect the meaning of the documentation.

Issues identified

1. Typo in schema-based sharding section

Current:
Multi-tenant apps can a use a schema per tenant to easily shard along the tenant dimension.

Suggested:
Multi-tenant apps can use a schema per tenant to easily shard along the tenant dimension.

2. Typo in feature list

Current:
References tables are replicated to all nodes for joins and foreign keys from distributed tables and maximum read performance.

Suggested:
Reference tables are replicated to all nodes for joins and foreign keys from distributed tables and to maximize read performance.

3. Missing preposition

Current:
To get started Citus on Azure, use the Azure Cosmos DB for PostgreSQL Quickstart.

Suggested:
To get started with Citus on Azure, use the Azure Cosmos DB for PostgreSQL Quickstart.

4. Grammar improvement

Current:
Some of you already start with Postgres, and decide to distribute tables later on while your application using the tables.

Suggested:
Some of you may start with Postgres and decide to distribute tables later while your application is already using the tables.

Proposed Fix

I can submit a pull request with these corrections if the maintainers agree.