Update README

README.md

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 [![Dev](https://img.shields.io/badge/docs-dev-blue.svg)](https://tensor4all.github.io/PartitionedMPSs.jl/dev)
 [![CI](https://github.com/tensor4all/PartitionedMPSs.jl/actions/workflows/CI.yml/badge.svg)](https://github.com/tensor4all/PartitionedMPSs.jl/actions/workflows/CI.yml)
+
+## General Information
+This library provides an implementation of partitioned matrix product states (MPSs) in Julia. A `PartitionedMPS` object is a collection of `SubDomainMPS` objects, each of which is an MPS projected on a subdomain of the whole index set.
+This library also provides a block-wise multiplication of partitioned MPSs.
+
+
+## Usage
+
+Please refer to the [documentation](https://tensor4all.github.io/PartitionedMPSs.jl/) for usage.