Add NEO4JLABS_PLUGINS to Docker developer guide

[adam-cowley]

docker run \
    --name testneo4j \
    -p7474:7474 -p7687:7687 \
    -d \
    -v $HOME/neo4j/data:/data \
    -v $HOME/neo4j/logs:/logs \
    -v $HOME/neo4j/import:/var/lib/neo4j/import \
    -v $HOME/neo4j/plugins:/plugins \
    --env NEO4JLABS_PLUGINS='["apoc", "neosemantics"]' # Installs APOC and Neosemantics
    --env NEO4J_AUTH=neo4j/test \
    neo4j:latest

https://github.com/neo4j/docker-neo4j/blob/master/neo4jlabs-plugins.json