Please add install instructions

[ghost]

Description
We initially tried to install the theia-csharp-extension which does not work yet correctly / at all.
So we stumbled over this plugin as alternative.
We installed it manually and fortunately it worked perfectly (although the popup sometimes takes a few seconds).
The setup was quite easy, like 1. creating a plugin directory, 2. placing this plugin either compressed as .theia file or uncompressed into the plugin directory, 3. starting theia with an additional commandline argument telling it where to load additional plugins.

What's Expected
Here is a brief info, how we used this extension in our docker container:

ENV PLUGIN_OMNISHARP_NAME=omnisharp_theia_plugin
ENV PLUGIN_OMNISHARP_VERSION=v0.0.6
ENV PLUGIN_OMNISHARP_URL=https://github.com/redhat-developer/omnisharp-theia-plugin/releases/download/${PLUGIN_OMNISHARP_VERSION}/${PLUGIN_OMNISHARP_NAME}.theia

USER theia

WORKDIR /home/theia

RUN mkdir --parents                             \
          ./plugins                             \
 && wget --output-document=/tmp/plugin.theia    \
         ${PLUGIN_OMNISHARP_URL}                \
 && unzip /tmp/plugin.theia                     \
          -d ./plugins/${PLUGIN_OMNISHARP_NAME} \
 && rm --force                                  \
       /tmp/plugin.theia

Tool versions
yarn: 1.16.0
npm: 6.7.0
node: v11.15.0
os: Ubuntu 18.04.2 LTS Desktop