Refer to Brian2 as Beta, not as Alpha
Add release notes for Brian 2.0beta
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Add "Problems with numerical integration" to the known issues
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Document linked variable mechanism
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Added warnings about potential problems with OpenMP code
Small doc fixes
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Final update of the number of bugs in the release notes
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Merge pull request #360 from brian-team/preparations_2.0beta

Preparations for 2.0beta