doc/contribute: add "Understanding the extension" section

Fixes issue 4996

R=golang-dev, r, bradfitz
CC=golang-dev
https://codereview.appspot.com/7547043

Committer: Rob Pike <r@golang.org>
io/ioutil: make WriteFile notice errors from Close()

WriteFile should not successfully return if the file's
Close call failed.

Fixes issue 5644.

R=golang-dev, kr, r
CC=golang-dev
https://codereview.appspot.com/10067043

Committer: Rob Pike <r@golang.org>
time: provide timezone abbreviations on windows

Use http://unicode.org/cldr/data/common/supplemental/windowsZones.xml
to generate the map.

Fixes issue 4838.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/9997043
Merge branch 'master' into gitfix