Adding Contributing file

[facebook-github-bot]

This is pull request was created automatically because we noticed your project was missing a Contributing file.

CONTRIBUTING files explain how a developer can contribute to the project - which you should actively encourage.

This PR was crafted with love by Meta's Open Source Team.

[github-actions[bot]]

workflow: benchmarks/size

Comparison of minified (terser) and compressed (brotli) size results, measured in bytes. Smaller is better.	Results	Base	Patch	Ratio
react-strict-dom/dist/dom/index.js
· compressed	2,488	2,488	1.00
· minified	8,586	8,586	1.00
react-strict-dom/dist/dom/runtime.js
· compressed	850	850	1.00
· minified	2,404	2,404	1.00
react-strict-dom/dist/native/index.js
· compressed	15,342	15,342	1.00
· minified	57,046	57,046	1.00

[github-actions[bot]]

workflow: benchmarks/perf (native)

Comparison of performance test results, measured in operations per second. Larger is better.	Results	Base	Patch	Ratio
css.create
· small	1,132,489	1,152,961	1.02	+
· small with units	443,408	449,975	1.01	+
· small with variables	683,587	678,846	0.99	-
· several small	325,428	329,233	1.01	+
· large	214,387	214,244	1.00	-
· large with polyfills	150,443	151,213	1.01	+
· complex	103,200	103,557	1.00	+
· unsupported	225,686	224,680	1.00	-
css.createTheme
· simple theme	221,503	220,070	0.99	-
· polyfill theme	209,210	208,987	1.00	-
css.props
· small	244,318	244,944	1.00	+
· small with units	186,869	188,050	1.01	+
· small with variables	106,528	106,195	1.00	-
· small with variables of units	75,273	74,428	0.99	-
· large	104,589	105,110	1.00	+
· large with polyfills	37,018	36,875	1.00	-
· complex	24,016	23,758	0.99	-
· unsupported	148,985	148,620	1.00	-
· simple merge	163,526	163,660	1.00	+
· wide merge	18,234	18,173	1.00	-
· deep merge	17,845	17,770	1.00	-
· themed merge	31,699	32,125	1.01	+

[necolas]

Contributing is thoroughly documented on the website, and linked to from the main README