add warning about manually installing PostCSS plugins

[pjaws]

Add a warning to the "Customizing PostCSS config" page instructing users to manually install any plugins they include in the file.

[ijjk]

Stats from current PR

Default Server Mode (Decrease detected ✓)

General

	zeit/next.js canary	pjaws/next.js canary	Change
buildDuration	11.1s	11.1s	⚠️ +84ms
nodeModulesSize	55.5 MB	55.5 MB	✓

Page Load Tests Overall decrease ⚠️

	zeit/next.js canary	pjaws/next.js canary	Change
/ failed reqs	0	0	✓
/ total time (seconds)	1.851	1.895	⚠️ +0.04
/ avg req/sec	1350.36	1319.34	-31.02
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	0.945	1.003	⚠️ +0.06
/error-in-render avg req/sec	2646.02	2492.99	-153.03

Client Bundles (main, webpack, commons)

	zeit/next.js canary	pjaws/next.js canary	Change
main-HASH.js gzip	6.25 kB	6.25 kB	✓
webpack-HASH.js gzip	746 B	746 B	✓
de003c3a9d30..5e5e.js gzip	10.3 kB	10.3 kB	✓
framework.a1..NSE.txt gzip	220 B	220 B	✓
framework.a1..NSE.txt gzip	220 B	220 B	✓
framework.HASH.js gzip	39 kB	39 kB	✓
Overall change	56.7 kB	56.7 kB	✓

Client Bundles (main, webpack, commons) Modern

	zeit/next.js canary	pjaws/next.js canary	Change
main-HASH.module.js gzip	4.79 kB	4.79 kB	✓
webpack-HASH..dule.js gzip	746 B	746 B	✓
de003c3a9d30..dule.js gzip	6.85 kB	6.85 kB	✓
framework.HA..dule.js gzip	39 kB	39 kB	✓
Overall change	51.4 kB	51.4 kB	✓

Legacy Client Bundles (polyfills)

	zeit/next.js canary	pjaws/next.js canary	Change
polyfills-HASH.js gzip	26.2 kB	26.2 kB	✓
Overall change	26.2 kB	26.2 kB	✓

Client Pages

	zeit/next.js canary	pjaws/next.js canary	Change
_app.js gzip	1.24 kB	1.24 kB	✓
_error.js gzip	3.12 kB	3.12 kB	✓
hooks.js gzip	664 B	664 B	✓
index.js gzip	222 B	222 B	✓
link.js gzip	2.06 kB	2.06 kB	✓
routerDirect.js gzip	279 B	279 B	✓
withRouter.js gzip	278 B	278 B	✓
Overall change	7.86 kB	7.86 kB	✓

Client Pages Modern

	zeit/next.js canary	pjaws/next.js canary	Change
_app.module.js gzip	602 B	602 B	✓
_error.module.js gzip	2.09 kB	2.09 kB	✓
hooks.module.js gzip	383 B	383 B	✓
index.module.js gzip	223 B	223 B	✓
link.module.js gzip	1.52 kB	1.52 kB	✓
routerDirect..dule.js gzip	281 B	281 B	✓
withRouter.m..dule.js gzip	278 B	278 B	✓
Overall change	5.38 kB	5.38 kB	✓

Client Build Manifests

	zeit/next.js canary	pjaws/next.js canary	Change
_buildManifest.js gzip	61 B	61 B	✓
_buildManife..dule.js gzip	61 B	61 B	✓
Overall change	122 B	122 B	✓

Rendered Page Sizes

	zeit/next.js canary	pjaws/next.js canary	Change
index.html gzip	927 B	927 B	✓
link.html gzip	937 B	937 B	✓
withRouter.html gzip	924 B	924 B	✓
Overall change	2.79 kB	2.79 kB	✓

---

Serverless Mode

General

	zeit/next.js canary	pjaws/next.js canary	Change
buildDuration	11.8s	11.9s	⚠️ +105ms
nodeModulesSize	55.5 MB	55.5 MB	✓

Client Bundles (main, webpack, commons)

	zeit/next.js canary	pjaws/next.js canary	Change
main-HASH.js gzip	6.25 kB	6.25 kB	✓
webpack-HASH.js gzip	746 B	746 B	✓
de003c3a9d30..5e5e.js gzip	10.3 kB	10.3 kB	✓
framework.a1..NSE.txt gzip	220 B	220 B	✓
framework.a1..NSE.txt gzip	220 B	220 B	✓
framework.HASH.js gzip	39 kB	39 kB	✓
Overall change	56.7 kB	56.7 kB	✓

Client Bundles (main, webpack, commons) Modern

	zeit/next.js canary	pjaws/next.js canary	Change
main-HASH.module.js gzip	4.79 kB	4.79 kB	✓
webpack-HASH..dule.js gzip	746 B	746 B	✓
de003c3a9d30..dule.js gzip	6.85 kB	6.85 kB	✓
framework.HA..dule.js gzip	39 kB	39 kB	✓
Overall change	51.4 kB	51.4 kB	✓

Legacy Client Bundles (polyfills)

	zeit/next.js canary	pjaws/next.js canary	Change
polyfills-HASH.js gzip	26.2 kB	26.2 kB	✓
Overall change	26.2 kB	26.2 kB	✓

Client Pages

	zeit/next.js canary	pjaws/next.js canary	Change
_app.js gzip	1.24 kB	1.24 kB	✓
_error.js gzip	3.12 kB	3.12 kB	✓
hooks.js gzip	664 B	664 B	✓
index.js gzip	222 B	222 B	✓
link.js gzip	2.06 kB	2.06 kB	✓
routerDirect.js gzip	279 B	279 B	✓
withRouter.js gzip	278 B	278 B	✓
Overall change	7.86 kB	7.86 kB	✓

Client Pages Modern

	zeit/next.js canary	pjaws/next.js canary	Change
_app.module.js gzip	602 B	602 B	✓
_error.module.js gzip	2.09 kB	2.09 kB	✓
hooks.module.js gzip	383 B	383 B	✓
index.module.js gzip	223 B	223 B	✓
link.module.js gzip	1.52 kB	1.52 kB	✓
routerDirect..dule.js gzip	281 B	281 B	✓
withRouter.m..dule.js gzip	278 B	278 B	✓
Overall change	5.38 kB	5.38 kB	✓

Client Build Manifests

	zeit/next.js canary	pjaws/next.js canary	Change
_buildManifest.js gzip	61 B	61 B	✓
_buildManife..dule.js gzip	61 B	61 B	✓
Overall change	122 B	122 B	✓

Serverless bundles

	zeit/next.js canary	pjaws/next.js canary	Change
_error.js	557 kB	557 kB	✓
404.html	4.18 kB	4.18 kB	✓
hooks.html	3.81 kB	3.81 kB	✓
index.js	557 kB	557 kB	✓
link.js	594 kB	594 kB	✓
routerDirect.js	586 kB	586 kB	✓
withRouter.js	586 kB	586 kB	✓
Overall change	2.89 MB	2.89 MB	✓

[timneutkens]

Thanks!