Add Slack badge and link in README (#1686)
Add Slack badge and link in README (#1686)

Signed-off-by: RALAMBOTIANA MIORA <b81a256d1a321c0675d676f02852ced68298e762@rte-france.com>
powsybl/powsybl-core,powsybl/powsybl-core,powsybl/powsybl-core