Add contributing section and license

Home.md

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 * **[Credits](https://github.com/AuroraWright/Luma3DS/wiki/Credits):** A credits page thanking numerous users who have contributed to the project over the years. Think we missed you? Let us know!
 
 If you're having trouble understanding anything on this wiki or need help using a feature, try asking for help in the [Nintendo Homebrew Discord](https://discord.gg/C29hYvh). See an error, inaccuracy, or etc. issue on the wiki? [Let us know so we can fix it!](https://github.com/AuroraWright/Luma3DS/issues) Your help in identifying problems is much appreciated.
+
+-------------------------
+
+# Contributing
+
+Contributions are welcome!
+
+Contributions should:
+- stay in-topic, that is, about Luma3DS and its features
+- neither mention nor encourage piracy
+
+To take screenshots of Rosalina menu, see [this commit](https://github.com/LumaTeam/Luma3DS/commit/ef1773ef4ad871dc29d43865aabcd3bb82006c66).
+
+Contributions are licensed under [Creative Commons Attribution-ShareAlike](https://creativecommons.org/licenses/by-sa/4.0/) going forward.

_Footer.md

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+Content is available under [Creative Commons Attribution-ShareAlike](https://creativecommons.org/licenses/by-sa/4.0/) unless otherwise noted.
+
 Take a look at the [FAQ and troubleshooting](https://github.com/AuroraWright/Luma3DS/wiki/FAQ-and-troubleshooting) page for the most common issues you might run into.
 For other issues or general support with using Luma3DS, ask for help at [Nintendo Homebrew on Discord](https://discord.gg/MjzatM8).
 To report issues, [create a Github issue](https://github.com/AuroraWright/Luma3DS/issues). Check to see if it was reported already before creating a new one.