Updated index.html

[YorhaBlue]

Fixes #568 Update broken links in index.html page

Line 157: changed "https://github.com/publiclab/plots2/blob/master/README.md" to "https://github.com/publiclab/plots2/blob/main/README.md"
Line 161: changed "https://github.com/publiclab/plots2/issues/new" to "https://github.com/publiclab/plots2/issues/new/choose"
Line 215: added href "https://publiclab.org/notes/warren/10-31-2016/create-a-welcoming-first-timers-only-issue-to-invite-new-software-contributors"

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[welcome]

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[on2onyekachi]

Great work! @YorhaBlue I see you made a separate PR for each file.

[YorhaBlue]

Thanks @on2onyekachi. But yes, sorry about the separate PRs -- still getting used to GitHub! I'll make sure to consolidate everything next time I work on an issue.

[on2onyekachi]

Okay, @YorhaBlue,

@TildaDares or other mentors will get to review this and merge it.

[TildaDares]

Hi @YorhaBlue, as @on2onyekachi said it'll be better if all of the changes related to this issue are in one PR. Thanks!!

[YorhaBlue]

Closing since I created PR #577 with the collective changes