Merge pull request #3277 from nspcc-dev/add-label-doc

docs: minimally outline local labels

docs/labels.md

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+# Project-specific labels
+
+## Component
+
+There is a number of internal components:
+
+- cli
+- compiler
+- consensus
+- network
+- oracle
+- rpc
+- smartcontract
+- vm
+- wallet
+
+## Platform
+
+In general we support Linux, Mac and Windows, but currently we use this:
+
+- windows
+  
+  It caused so many problems that it deserves a label of its own.