Readme important update

[miguepollo]

In the readme i have changed the pip of the system to the venv pip. And i have changed the file name requirments to requirements.

[zhengxyz123]

Actually, changing system pip to venv pip is unnecessary.

You can run the following lines in bash:

$ which pip
/usr/bin/pip
$ . bin/active
<venv path>/bin/pip

The requirments is my mistake. Thank you!