Grammar fixes

[cobrexus]

Some grammar fixes in CONTRIBUTING.md and README.md, and improvements to sentences structure.

[halfmoonui]

Thank you for the PR. One issue is after your change, one statement reads as follows:

Halfmoon's stated goal is really simple - create a customizable frontend framework, has a built-in dark mode, and is great for building dashboards and tools. However, other goals are more implicit:

It doesn't seem correct grammatically. There needs to be a that before has a built-in dark mode.

[cobrexus]

@halfmoonui I've updated that line to say:

Halfmoon's stated goal is really simple - create a customizable frontend framework, with a built-in dark mode, that is brilliant for building dashboards and tools. However, our other goals are more implicit: