contributing.md: Add CONTRIBUTING.md
contributing.md: Add CONTRIBUTING.md

As discussed in the monthly OE4T meeting, move the contributing
guide into the master branch of meta-tegra.

This content includes the latest from the wiki page as of 2020-12-12

Signed-off-by: Dan Walkes <1853477f5849bd091a945c4672065016fd4c2842@boulderai.com>

madisongh/meta-tegra,madisongh/meta-tegra,madisongh/meta-tegra,madisongh/meta-tegra,madisongh/meta-tegra