[master] Fix README to better state to add not to create an already existing file.
[master] Fix README to better state to add not to create an already existing file.

alvarolobato/blueocean-plugin,kzantow/blueocean-plugin,jenkinsci/blueocean-plugin,alvarolobato/blueocean-plugin,jenkinsci/blueocean-plugin,tfennelly/blueocean-plugin,ModuloM/blueocean-plugin,jenkinsci/blueocean-plugin,tfennelly/blueocean-plugin,tfennelly/blueocean-plugin,tfennelly/blueocean-plugin,alvarolobato/blueocean-plugin,ModuloM/blueocean-plugin,jenkinsci/blueocean-plugin,kzantow/blueocean-plugin,kzantow/blueocean-plugin,alvarolobato/blueocean-plugin,ModuloM/blueocean-plugin,ModuloM/blueocean-plugin,jenkinsci/blueocean-plugin,kzantow/blueocean-plugin