[Doc] Add direct pip installation command in README.md

[mjhong0708]

Thanks for such a great library.

Since pip allows direct installation from remote git repository by running following command,

user@host:~$ pip install git+https://github.com/jewettaij/moltemplate.git

it will be very helpful for new users to add this in README.md, especially who's not familiar with python.

[jewettaij]

Hi Minjoon

Thanks for letting me know about this neat trick! This will help users who are unfamiliar with git. I think the existing instructions were really lacking, and your suggestion helped me realize this. However I do want to encourage users to download the entire moltemplate repository because it includes a lot of documentation and examples that they will miss if they only use pip.

So I added some instructions how to use git to download moltemplate before using pip. Then I also added your suggestion as well ("pip install git+https://github.com/jewettaij/moltemplate.git"), which allows users to avoid using pip.

(On a side note, perhaps it makes more sense to put all the documentation and examples in a separate repository. I tried that with a different program I wrote, but only a few users (<10%) ever bother to download the examples (located in a separate repository).)

Anyway, thank you for your suggestion! I think it will be helpful to people.