Add GitHub SUPPORT file (micrometer-metrics#1290)

SUPPORT.adoc

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+= Getting support for Micrometer
+
+== GitHub issues
+We choose not to use GitHub issues for general usage questions and support, preferring to
+use issues solely for the tracking of bugs and enhancements. If you have a general
+usage question please do not open a GitHub issue, but use one of the other channels
+described below.
+
+If you are reporting a bug, please help to speed up problem diagnosis by providing as
+much information as possible. Ideally, that would include a small sample project that
+reproduces the problem.
+
+== Stack Overflow
+The Micrometer community monitors the
+https://stackoverflow.com/tags/micrometer[`micrometer`] tag on Stack Overflow. Before
+asking a question, please familiarize yourself with Stack Overflow's
+https://stackoverflow.com/help/how-to-ask[advice on how to ask a good question].
+
+== Slack
+If you want to discuss something or have a question that isn't suited to Stack Overflow,
+the Micrometer community chat in the
+http://slack.micrometer.io/[micrometer-metrics channel on Slack].