Update gitbook "Developers" section

[dcts]

@Radstakes can you update this section of the gitbook:

https://dexter-on-radix.gitbook.io/dexter/overview/how-do-i-contribute#developers

My suggestion:

Developers

Interested in contributing to our code? Explore all our public repositories here.

For our webapp, comment on any issue labeled "help wanted" and we'll assign it to you and provide any necessary onboarding assistance:

Open Issues

[yuliaSharabi]

I would suggest to add a sentence in the following mode: "If you are a beginner, you can always use our "good first issue" label.