Readme update

[LMJ-19]

While creating a beta competition example snippet we (mostly Martijn) noticed some small typo's / things that could be updated in the snippet:

in the readme:

• So build and see the docs see the [docs README](docs/README.md).: So --> To
• Copyright 2018 QuTech (TUDelft and TNO): 2018 --> 2020

in the docs/readme:

• which makes use of the command open. If you're on Linux and open does not work you can add alias open='xdg-open to you rc-file.: missing an ' after 'xdg-open

in requirements.txt:

• it would be good to add pip>=19.0, since we tested with a VirtualEnv which defaults with pip v9 which does not work with NetSquid

in cookiecutter.json:

• the default NetSquid requirement is ">=0.8.0,<0.9.0", with the beta release we coud (or should) update this to ">=0.9.0,<1.0.0". In the example snippet I need netsquid>=0.9.1

[LMJ-19]

in install.md:

• There are the general installation instructions for NetSquid snippet packages.: There --> These