Add a release checklist (#324)

* Add a release checklist

Author: fzhinkin

docs/ReleaseChecklist.md

@@ -0,0 +1,41 @@
+# kotlinx-io release check list
+
+`kotlinx-io` release process consists of several partially automated steps.
+
+This document is aimed for engineers responsible for releasing the library and should read as an 
+overview of how things are done usually rather than a rigid script that should be followed strictly.
+
+## TODO
+
+Some steps are not fully automated, some steps fail (like the smoke tests) even though everything
+is fine. It all should be fixed sooner or later.
+
+## Checklist
+
+As it is described in [contibuting guidelines](../CONTRIBUTING.md), 
+the developent takes place in `develop` branch
+and `master` branch contains sources for a recently released version.
+
+The following steps are usually performed:
+- Ensure all tests pass in CI for the `develop` branch;
+- Update [README](../README.md) instructions on how to use the library with a new version;
+- Add a note describing what will be released to the [CHANGELOG](../CHANGELOG.md);
+- Bump up the current version in [gradle.properties](../gradle.properties);
+- Commit changes in `README`, `CHANGELOG` and `gradle.properties` into the `develop` branch;
+- Checkout `gh-pages` branch, generate documentation using Dokka (`./gradlew dokkaHtmlMultiModule`), move it to `docs` directory, and commit it all;
+  - that generated docs use correct `kotlinx-io` version;
+- [Deploy](https://teamcity.jetbrains.com/buildConfiguration/KotlinTools_KotlinxIo_DeployRunThisOne)
+a pre-release version of the library into [Sonatype](http://oss.sonatype.org) staging repository,
+close it and [run smoke tests](https://teamcity.jetbrains.com/buildConfiguration/KotlinTools_KotlinxIo_DeploymentSmokeTest) 
+with this pre-release version;
+- If smoke tests passed, merge the `develop` branch into `master`, wait until a successful test run in CI;
+- Drop the pre-release version from Sonatype;
+- [Create a draft](https://github.com/Kotlin/kotlinx-io/releases/new) describing the next release in GH:
+  - it's more or less fine to pick up the record from the [CHANGELOG](../CHANGELOG.md), update it if
+  necessary;
+  - release names are usually starts with `v`, like `v0.9.10`, but there's no `v` in a git tag, like `0.9.10`.
+- Deploy the release version, close the repo, run smoke tests with it and if tests passed, release the repository.
+- Push `gh-pages` branch;
+- Set previously created draft in [kotlinx-io/releases](https://github.com/Kotlin/kotlinx-io/releases)
+as the latest version;
+- That's all, folks!