README.md: fix indentation
README.md: fix indentation

This change also tweaks the wording from "many" to "some" with respect
to non-working ImageJ1 plugins, since we are getting closer to all
ImageJ1 functionality being either functional or ported to ImageJ2.

TehSAUCE/imagej,biovoxxel/imagej,TehSAUCE/imagej,biovoxxel/imagej,biovoxxel/imagej,TehSAUCE/imagej