golang plugin package dependency requires 1.8

[mikebrow]

Minor update to README.md to mention the 1.8 requirement.. otherwise one will receive the following build error:

mike@mike-VirtualBox:~/go/src/github.com/docker/containerd$ make binaries
🐳 bin/ctr
plugin.go:6:2: cannot find package "plugin" in any of:
	/home/mike/go/src/github.com/docker/containerd/vendor/plugin (vendor tree)
	/usr/local/go/src/plugin (from $GOROOT)
	/home/mike/go/src/plugin (from $GOPATH)

Signed-off-by: Mike Brown brownwm@us.ibm.com

[codecov-io]

Codecov Report

Merging #564 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #564   +/-   ##
=======================================
  Coverage   62.12%   62.12%           
=======================================
  Files           3        3           
  Lines         404      404           
=======================================
  Hits          251      251           
  Misses         93       93           
  Partials       60       60

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[stevvooe]

LGTM