add note about maven plugins

[macjohnny]

PR checklist

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• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
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Description of the PR

add note about maven plugin

[jmini]

Maybe we should create an explicit section "1.2 - Maven Coordinates" before "1.2 - Download JAR" (and then of course reorder the other sections)

Similar to what we have in the migration guide, new maven coordinates

Something like (with nicer links as you did and link to the corresponding readme when they exists):

---

You can find our released artifacts on maven central:

Core:

<dependency>
    <groupId>org.openapitools</groupId>
    <artifactId>openapi-generator</artifactId>
    <version>${openapi-generator-version}</version>
</dependency>

https://mvnrepository.com/artifact/org.openapitools/openapi-generator

Cli:

<dependency>
    <groupId>org.openapitools</groupId>
    <artifactId>openapi-generator-cli</artifactId>
    <version>${openapi-generator-version}</version>
</dependency>

https://mvnrepository.com/artifact/org.openapitools/openapi-generator-cli

Maven plugin:

<dependency>
    <groupId>org.openapitools</groupId>
    <artifactId>openapi-generator-maven-plugin</artifactId>
    <version>${openapi-generator-version}</version>
</dependency>

https://mvnrepository.com/artifact/org.openapitools/openapi-generator-maven-plugin

https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator-maven-plugin/README.adoc

Gradle plugin:

<dependency>
    <groupId>org.openapitools</groupId>
    <artifactId>openapi-generator-gradle-plugin</artifactId>
    <version>${openapi-generator-version}</version>
</dependency>

https://mvnrepository.com/artifact/org.openapitools/openapi-generator-gradle-plugin

https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator-gradle-plugin/README.adoc

[macjohnny]

@jmini thanks for your suggestions, I updated the PR accordingly

[wing328]

Looks good to me (personally I would prefer adding a new section as renaming existing sections may break the browser bookmark)