Feat/update docs

[semangard]

Motivation

Update doc

What

Add explanations about how to use the env var PROMETHEUS_PUSHGATEWAY_JOB

Why

Missing into the doc

How

Update README.md

Verification Steps

None

Checklist:

• [X ] Code has been tested locally by PR requester
• [ ] Changes have been successfully verified by another team member

Progress

• [x] Finished task
• [ ] TODO

[semangard]

Is related to previous PR : https://github.com/aerogear/keycloak-metrics-spi/pull/102 The update of the doc was missing

[semangard]

@pb82 : hello, it is an easy one, coudl you please merge it ? (thanks and regards)

[pb82]

thanks @semangard !