Added a couple of known issues

[astrofrog]

For #3415 and #2960 (comment)

cc @embray @eteq @olebole @bsipocz

[eteq]

@astrofrog - this is fine, but while you're editing the known issues, can I suggest two other changes?:

1. Re-arrange the issues so that some of the more common installation issues are at the top. Specifically, the conda/pip issue, and the locale issue. Also, perhaps move some of the issues dealing with older versions further down? (esp. the Astropy v0.2 issue, but possibly also the py 2.6.x-specific one)
2. Can you add a table-of-contents to the top? I'm not entirely sure this will work, but a .. toctree:: directive pointing to the current file might be the way to go. I realize this appears in the sidebar in the generated docs, but I think it might still be helpful to see it inline.

[astrofrog]

@eteq - done, and here's the current order (suggestions welcome):

[eteq]

Looks good to me except for one other minor change I'd like to suggest: "Local Errors" -> "Locale Errors in Mac OS X".

[Cadair]

@eteq I have seen the locale errors on Linux as well.

[astrofrog]

@Cadair - ok, fixed

[eteq]

This looks good now, so I'll go ahead and merge.

[eteq]

Backported to v1.0.x in fd33850