Adjust scrollbar display after layout if autosized children

ScrollableControl.OnLayout sets the scroll bar display before doing the
actual layout.  If any children are autosized, they may change size as
a result of the layout, and that may change whether or not the scrollbar
is needed.  (This addresses Xamarin bug 18874.)
[llvm] Fix a few problems with longs on 32 bit platforms. Fixes #18925.
Added contribution guideline links in README 

New section has links to guideline wiki page, and the page's Important Rules section that all contributors should be aware of.
Merge pull request #987 from StephenMcConnel/bug-18874

Adjust scrollbar display after layout if autosized children