Add README info

[yo1995]

Add a sentence to the Issues section in README to explain that the toolkit is open source and it is okay to encourage users to customize the toolkit as they need to. Use what we provide as a starting point.

For example, they can

• copy paste source code into their project directly
• fork the repo, make their own edits, tag their own releases, and merge from upstream updates when we release

[mhdostal]

Maybe include a link to the Wikipedia open source definition?

[chriswebb09]

@mhdostal @yo1995 I can take this one.

[dfeinzimer]

@yo1995 / @chriswebb09 are we okay to close this out since #1289 was closed, or did we still want to give this consideration?