RFR: Added gpg plugin and steps for release

[Jiří Vaněk]

On Wed, 9 Jun 2021 12:03:54 GMT, Jiří Vaněk <jvanek at openjdk.org> wrote:

> Usually the maven release plugin or gpg release plugin are working with direct access to repo - push change of versions, tag, do release, and again bump versions for next development cycle. As for asmtools, the maven is just secondary, unsupported release medium so I had opted for upload of locally finished binaries. Thus the manual ```mvn clean verify gpg:sign install:install deploy:deploy -DaltDeploymentRepository=ossrh::default::https://oss.sonatype.org/service/local/staging/deploy/maven2/ ```  command in pom comment. 
> This may not be best, but for now it is the only way I found to upload locally created binaries. But I must admit, I'm not 100% sure in maven reelase or gpg plugins. During the next releases, maybe I will find something better.
> 
> As the PR is now, the asmtools artifacts are uploaded in staging maven central: https://oss.sonatype.org/content/repositories/orgopenjdkasmtools-1002/org/openjdk/asmtools/asmtools-core/
> 
> I had tested it and it works as expected (https://github.com/pmikova/java-runtime-decompiler/commit/d8b5a03759b9e8dfe584b0052778ff590ba6e82b).
> 
> One offtopic question  - the {slash}integrate and/or {slash}sponsor is autoamtically squashing commits?

I will move from staging to maven central once this PR is merged.

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PR: https://git.openjdk.java.net/asmtools/pull/20