Update contributing.md to link to walkthrough video.

[leerob]

Hopefully this helps folks contribute 😄

[ijjk]

Stats from current PR

Default Build (Decrease detected ✓)

General Overall decrease ✓

	vercel/next.js canary	vercel/next.js leerob-patch-1	Change
buildDuration	15.2s	15.1s	-66ms
buildDurationCached	6s	6s	-2ms
nodeModulesSize	359 MB	359 MB	-3 B

Page Load Tests Overall decrease ⚠️

	vercel/next.js canary	vercel/next.js leerob-patch-1	Change
/ failed reqs	0	0	✓
/ total time (seconds)	3.019	3.015	0
/ avg req/sec	827.97	829.06	+1.09
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.185	1.187	0
/error-in-render avg req/sec	2109.34	2106.6	⚠️ -2.74

Client Bundles (main, webpack, commons)

	vercel/next.js canary	vercel/next.js leerob-patch-1	Change
450.HASH.js gzip	179 B	179 B	✓
framework-HASH.js gzip	42 kB	42 kB	✓
main-HASH.js gzip	27.9 kB	27.9 kB	✓
webpack-HASH.js gzip	1.44 kB	1.44 kB	✓
Overall change	71.5 kB	71.5 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	vercel/next.js leerob-patch-1	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	vercel/next.js leerob-patch-1	Change
_app-HASH.js gzip	1.36 kB	1.36 kB	✓
_error-HASH.js gzip	194 B	194 B	✓
amp-HASH.js gzip	312 B	312 B	✓
css-HASH.js gzip	326 B	326 B	✓
dynamic-HASH.js gzip	2.57 kB	2.57 kB	✓
head-HASH.js gzip	350 B	350 B	✓
hooks-HASH.js gzip	919 B	919 B	✓
image-HASH.js gzip	5.01 kB	5.01 kB	✓
index-HASH.js gzip	263 B	263 B	✓
link-HASH.js gzip	2.26 kB	2.26 kB	✓
routerDirect..HASH.js gzip	321 B	321 B	✓
script-HASH.js gzip	383 B	383 B	✓
withRouter-HASH.js gzip	318 B	318 B	✓
85e02e95b279..7e3.css gzip	107 B	107 B	✓
Overall change	14.7 kB	14.7 kB	✓

Client Build Manifests

	vercel/next.js canary	vercel/next.js leerob-patch-1	Change
_buildManifest.js gzip	459 B	459 B	✓
Overall change	459 B	459 B	✓

Rendered Page Sizes

	vercel/next.js canary	vercel/next.js leerob-patch-1	Change
index.html gzip	532 B	532 B	✓
link.html gzip	545 B	545 B	✓
withRouter.html gzip	527 B	527 B	✓
Overall change	1.6 kB	1.6 kB	✓

---

Default Build with SWC (Increase detected ⚠️)

General Overall decrease ✓

	vercel/next.js canary	vercel/next.js leerob-patch-1	Change
buildDuration	18.6s	18.5s	-105ms
buildDurationCached	6s	5.9s	-56ms
nodeModulesSize	359 MB	359 MB	-3 B

Page Load Tests Overall increase ✓

	vercel/next.js canary	vercel/next.js leerob-patch-1	Change
/ failed reqs	0	0	✓
/ total time (seconds)	3.048	2.984	-0.06
/ avg req/sec	820.15	837.78	+17.63
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.174	1.165	-0.01
/error-in-render avg req/sec	2129.96	2145.89	+15.93

Client Bundles (main, webpack, commons)

	vercel/next.js canary	vercel/next.js leerob-patch-1	Change
450.HASH.js gzip	179 B	179 B	✓
framework-HASH.js gzip	42.1 kB	42.1 kB	✓
main-HASH.js gzip	27.9 kB	27.9 kB	✓
webpack-HASH.js gzip	1.44 kB	1.44 kB	✓
Overall change	71.7 kB	71.7 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	vercel/next.js leerob-patch-1	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	vercel/next.js leerob-patch-1	Change
_app-HASH.js gzip	1.35 kB	1.35 kB	✓
_error-HASH.js gzip	180 B	180 B	✓
amp-HASH.js gzip	305 B	305 B	✓
css-HASH.js gzip	321 B	321 B	✓
dynamic-HASH.js gzip	2.56 kB	2.56 kB	✓
head-HASH.js gzip	342 B	342 B	✓
hooks-HASH.js gzip	911 B	911 B	✓
image-HASH.js gzip	5.05 kB	5.05 kB	✓
index-HASH.js gzip	256 B	256 B	✓
link-HASH.js gzip	2.28 kB	2.28 kB	✓
routerDirect..HASH.js gzip	314 B	314 B	✓
script-HASH.js gzip	375 B	375 B	✓
withRouter-HASH.js gzip	309 B	309 B	✓
85e02e95b279..7e3.css gzip	107 B	107 B	✓
Overall change	14.7 kB	14.7 kB	✓

Client Build Manifests

	vercel/next.js canary	vercel/next.js leerob-patch-1	Change
_buildManifest.js gzip	458 B	458 B	✓
Overall change	458 B	458 B	✓

Rendered Page Sizes

	vercel/next.js canary	vercel/next.js leerob-patch-1	Change
index.html gzip	533 B	533 B	✓
link.html gzip	546 B	546 B	✓
withRouter.html gzip	526 B	526 B	✓
Overall change	1.6 kB	1.6 kB	✓

Commit: d3e7039