docs: minimally outline local labels

Fixes #2707.

Signed-off-by: Roman Khimov <roman@nspcc.ru>

docs/labels.md

@@ -0,0 +1,23 @@
+# 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.