Add UPGRADING doc

[Timer]

This adds an UPGRADING.md document.

[github-actions]

Stats from current PR

Click to expand stats

	zeit/next.js canary	Timer/next.js upgrading-doc	Change
Build Duration	12.2s	12.4s	⚠️ +261ms
node_modules Size	45.4 MB	45.4 MB	⚠️ +4 B
Total Bundle (main, webpack, commons) Size	210 kB	210 kB	✓
Total Bundle (main, webpack, commons) gzip Size	68.9 kB	68.9 kB	✓
Client _app Size	2.39 kB	2.39 kB	✓
Client _app gzip Size	1.08 kB	1.08 kB	✓
Client _error Size	8.45 kB	8.45 kB	✓
Client _error gzip Size	3.24 kB	3.24 kB	✓
Client pages/index Size	296 B	296 B	✓
Client pages/index gzip Size	222 B	222 B	✓
Client pages/link Size	4.76 kB	4.76 kB	✓
Client pages/link gzip Size	2.03 kB	2.03 kB	✓
Client pages/routerDirect Size	423 B	423 B	✓
Client pages/routerDirect gzip Size	306 B	306 B	✓
Client pages/withRouter Size	435 B	435 B	✓
Client pages/withRouter gzip Size	301 B	301 B	✓
Client main Size	24.3 kB	24.3 kB	✓
Client main gzip Size	7.97 kB	7.97 kB	✓
Client commons Size	183 kB	183 kB	✓
Client commons gzip Size	59.6 kB	59.6 kB	✓
Client webpack Size	1.49 kB	1.49 kB	✓
Client webpack gzip Size	770 B	770 B	✓
Base Rendered Size	1.36 kB	1.36 kB	✓
Build Dir Size	786 kB	786 kB	✓

Click to expand serverless stats

	zeit/next.js canary	Timer/next.js upgrading-doc	Change
Build Duration	13.8s	14.2s	⚠️ +375ms
node_modules Size	45.4 MB	45.4 MB	⚠️ +4 B
Total Bundle (main, webpack, commons) Size	210 kB	210 kB	✓
Total Bundle (main, webpack, commons) gzip Size	68.9 kB	68.9 kB	✓
Client _app Size	2.39 kB	2.39 kB	✓
Client _app gzip Size	1.08 kB	1.08 kB	⚠️ +1 B
Client _error Size	8.45 kB	8.45 kB	✓
Client _error gzip Size	3.24 kB	3.24 kB	✓
Client pages/index Size	296 B	296 B	✓
Client pages/index gzip Size	224 B	222 B	-2 B
Client pages/link Size	4.76 kB	4.76 kB	✓
Client pages/link gzip Size	2.03 kB	2.03 kB	✓
Client pages/routerDirect Size	423 B	423 B	✓
Client pages/routerDirect gzip Size	306 B	306 B	✓
Client pages/withRouter Size	435 B	435 B	✓
Client pages/withRouter gzip Size	300 B	301 B	⚠️ +1 B
Client main Size	24.3 kB	24.3 kB	✓
Client main gzip Size	7.97 kB	7.97 kB	⚠️ +1 B
Client commons Size	183 kB	183 kB	✓
Client commons gzip Size	59.6 kB	59.6 kB	✓
Client webpack Size	1.49 kB	1.49 kB	✓
Client webpack gzip Size	770 B	770 B	✓
Serverless pages/link Size	338 kB	338 kB	✓
Serverless pages/link gzip Size	86.9 kB	86.9 kB	-1 B
Serverless pages/index Size	Error getting size	Error getting size	✓
Serverless pages/index gzip Size	Error getting size	Error getting size	✓
Serverless pages/_error Size	329 kB	329 kB	✓
Serverless pages/_error gzip Size	84.1 kB	84.1 kB	✓
Serverless pages/routerDirect Size	330 kB	330 kB	✓
Serverless pages/routerDirect gzip Size	84.4 kB	84.4 kB	-1 B
Serverless pages/withRouter Size	330 kB	330 kB	✓
Serverless pages/withRouter gzip Size	84.4 kB	84.4 kB	✓
Build Dir Size	2.04 MB	2.04 MB	✓