Create RELEASING.md (#19)

* Create RELEASING.md

* Update RELEASING.md

Author: bsneed

RELEASING.md

@@ -0,0 +1,9 @@
+Releasing
+=========
+
+1. Update the version in Sources/Version.swift
+2. `git commit -am "Version X.Y.Z"`
+3. `git tag -a X.Y.Z -m "Version X.Y.Z"`
+4. `git push && git push --tags`
+5. Create a new github release at https://github.com/segmentio/analytics-swift/releases
+    * Summarize change history since last release in the notes.