Document how to make `apt-get build-dep python3.5` to work

[brettcannon]

I.e. if you get the error about needing to update sources.list, then point out how to do that.

[brettcannon]

http://askubuntu.com/questions/496549/error-you-must-put-some-source-uris-in-your-sources-list, although on 14.04 I just copied the three lines I had and then changed deb to deb-src.

[warsaw]

I don't see a diff, but that should pretty much work.

[brettcannon]

@warsaw you "don't see a diff" as in you "don't see a diff file", or you "don't see a difference"? If it's the former it's because I just didn't have the time to do a PR. If it's the latter, not sure what you mean other than maybe not explaining exactly and instead just linking to the askubuntu.com link?

[DimitrisJim]

Just bumped into this myself on 16.04 and had to comment out a couple of deb-srcs. Would adding a url to the question on ask-ubuntu suffice? Or would a simple line of the form "Back up your /etc/apt/sources.list and then uncomment the deb_src lines" do the trick?

[brettcannon]

Depends on how easy it is for that question to becomes outdated in the future. If it's easy then listing the individual steps would be good, but if the answer is stable then linking to it should suffice.

[louisom]

I just encounter the same problem today on 17.04, and the error says that must update source.list. And resolve by manually install all dependency files. (the method list here isn't quite easy to found at Internet...)