Add instructions to download from GitHub and how to make a contribution

[riccardoleone1991]

Closes #718 .

Changes proposed in this pull request:

• Added helpful notes in dev_guide.rst

[welcome]

Thanks for opening this pull request! We have detected this is the first time you have contributed to NiMARE. Please check out our contributing guidelines.
We invite you to list yourself as a NiMARE contributor, so if your name is not already mentioned, please modify the .zenodo.json file with your data right above Angie's entry. Example:

{
  "name": "Contributor, New",
  "affiliation": "Department of Psychology, Some University",
  "orcid": "<your id>"
},
{
  "name": "Laird, Angela R.",
  "affiliation": "Florida International University",
  "orcid": "0000-0003-3379-8744"
},

Of course, if you want to opt out this time there is no problem at all with adding your name later. You will be always welcome to add it in the future whenever you feel it should be listed.

[codecov]

Codecov Report

Merging #720 (25dd735) into main (cb542af) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #720   +/-   ##
=======================================
  Coverage   85.34%   85.34%           
=======================================
  Files          41       41           
  Lines        4539     4539           
=======================================
  Hits         3874     3874           
  Misses        665      665           

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[jdkent]

need to change hyperlink, otherwise LGTM

[tsalo]

I think creating a fork and cloning that is a necessary step for folks who want to install NiMARE for development.