Add pre-release checklist

Checklist

+Before release:
+
+1. Check appropriate issues are fixed
+
+    https://github.com/haskell/time/pulls
+    https://github.com/haskell/time/issues
+
+2. Update version numbers
+
+    https://pvp.haskell.org/
+    time.cabal
+    configure.ac
+
+3. Update changelog
+
+    changelog.md
+
+4. Use latest LTS resolver
+
+    https://www.stackage.org/lts
+    time.cabal
+    (not necessarily benchmark/time-bench.cabal)
+
+5. Build & test
+
+    stack build --pedantic --test --haddock && echo OK
+
+6. Build benchmark
+
+    cd benchmark
+    stack build --pedantic
+        (ignore missing modules warning)
+    stack exec -- time-bench
+
+7. Read through generated haddock
+
+    .stack-work/install/[PLATFORM]/[RESOLVER]/[COMPILER]/doc/index.html
+
+8. Commit and push changes to repo
+
+    git commit -a
+    git push
+
+9. Check Travis build
+
+    https://travis-ci.org/haskell/time
+
+10. Build and test on Windows
+
+    git pull
+    stack build --pedantic --test --haddock && echo OK
+
+11. Upload to Hackage
+
+    stack upload .
+    http://hackage.haskell.org/package/time
+
+12. Tag commit
+
+    git tag -a -s [TAG]
+    git push --tags