docs: Describe the release procedure (#2104)

* docs: Describe the release procedure

* Add note about gradle

* Address comments

docs/release.md

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+# Appium Java Client Release Procedure
+
+This document describes the process of releasing this client to the Maven repository.
+Its target auditory is project maintainers.
+
+## Release Steps
+
+1. Update the [Changelog](../CHANGELOG.md) for the given version based on previous commits.
+1. Bump the `appiumClient.version` number in [gradle.properties](../gradle.properties).
+1. Create a pull request to approve the changelog and version bump.
+1. Merge the pull request after it is approved.
+1. Create and push a new repository tag. The tag name should look like 
+ `v<major_number>.<minor_number>.<patch_number>`. 
+1. Open [Sonatype](https://oss.sonatype.org/#welcome) in your browser.
+1. Login to Nexus using 1Password credentials. Ask Appium maintainers
+ if you need access to the team's 1Password vault.
+1. Navigate to `Staging Repositories`. 
+1. Select the corresponding release and click `Close`.
+1. Wait until checks are completed.
+1. Click `Release`.
+1. After the new release is published, it becomes available in 
+ [Maven Central](https://repo1.maven.org/maven2/io/appium/java-client/)
+ within 30 minutes. Once artifacts are in Maven Central, it normally 
+ takes 1-2 hours before they appear in 
+ [search results](https://central.sonatype.com/artifact/io.appium/java-client).