refine README to include installation and add release.sh

[chaozbj]

The following changes are for our first release:

1. Refine README.doc to include installation part
2. Add release.sh to build kn-admin for MacOS, Linux and Windows.
3. Follow the kn client rules to generate version for kn-admin, please see the below examples.

Currently, we still use module knative.dev/client-contrib/plugins/admin as module name in our codes. It is maybe changed because we moved into a new repository. I will keep eyes on that and change the release.sh as the module name is changed.

• Here is the example of release.sh run:

  ~/chaozbj/kn-plugin-admin/cmd [chao-refine-readme ✘] ./release.sh
  🚧 🍏 Building for macOS
  🚧 🐧 Building for Linux
  🚧 🎠 Building for Windows

• Here is the binaries generated by release.sh for all supported platforms:

  ~/chaozbj/kn-plugin-admin/cmd [chao-refine-readme ✘] ls -l
  total 265064
  -rwxr-xr-x  1 chao  staff  44959976 Oct 19 08:35 kn-admin-darwin-amd64
  -rwxr-xr-x  1 chao  staff  45356225 Oct 19 08:35 kn-admin-linux-amd64
  -rwxr-xr-x  1 chao  staff  45247488 Oct 19 08:35 kn-admin-windows-amd64
  -rw-r--r--  1 chao  staff       870 Oct 19 07:52 kn-admin.go
  -rwxr-xr-x  1 chao  staff      1414 Oct 19 08:26 release.sh

• Here is the version outputted by kn-admin:

  ~/chaozbj/kn-plugin-admin/cmd [chao-refine-readme ✘] ./kn-admin-darwin-amd64 version
  Version:      v20201019-e8bf703
  Build Date:   2020-10-19 00:35:40
  Git Revision: e8bf703

[knative-prow-robot]

Welcome @chaozbj! It looks like this is your first PR to knative-sandbox/kn-plugin-admin 🎉

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chaozbj, rhuss

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