minor updates on README.md

## Installation

Reliable download is developed and tested on linux. Windows and MacOS is not

Reliable download is developed and tested in linux. Windows and MacOS is not

supported. Theoretically the code can be ported to support other OS, but I

don't have the time to handle testing and distribution.

Reliable download server side and client side is both written in

Haskell. However, it is distributed on [pypi](https://pypi.org/) so that user

can install it more easily. Because python and pip is usually bundled with

linux system. To install rd-api and rd, see their

[pypi page](https://pypi.org/project/rd-api/). The same doc is kept in git as

well, see `./pypi/rd-api/README.rst`.

linux system. To install rd-api and rd, see their pypi page below. The same

doc is kept in git as well, see `./pypi/rd-api/README.rst`.

- [rd-api pypi page](https://pypi.org/project/rd-api/)

- [rd pypi page](https://pypi.org/project/rd/)

- [rd-api on pypi](https://pypi.org/project/rd-api/)

- [rd on pypi](https://pypi.org/project/rd/)

## Design and Command Line Help