Gemfile: add note about upgrade process of github-pages gem (#1707)
Gemfile: add note about upgrade process of github-pages gem (#1707)

The Gemfile cannot be upgraded to a newer version of the github-pages
gem without having a corresponding tag in the starefossen/github-pages
image.
phiroict/docker,thaJeztah/docker.github.io,bdwill/docker.github.io,thaJeztah/docker.github.io,johnstep/docker.github.io,joaofnfernandes/docker.github.io,shubheksha/docker.github.io,docker-zh/docker.github.io,londoncalling/docker.github.io,docker-zh/docker.github.io,JimGalasyn/docker.github.io,docker-zh/docker.github.io,shubheksha/docker.github.io,phiroict/docker,joeuo/docker.github.io,docker/docker.github.io,shin-/docker.github.io,joaofnfernandes/docker.github.io,anweiss/docker.github.io,londoncalling/docker.github.io,LuisBosquez/docker.github.io,thaJeztah/docker.github.io,thaJeztah/docker.github.io,bdwill/docker.github.io,docker/docker.github.io,shin-/docker.github.io,joeuo/docker.github.io,bdwill/docker.github.io,JimGalasyn/docker.github.io,phiroict/docker,docker-zh/docker.github.io,phiroict/docker,shubheksha/docker.github.io,anweiss/docker.github.io,JimGalasyn/docker.github.io,anweiss/docker.github.io,thaJeztah/docker.github.io,bdwill/docker.github.io,bdwill/docker.github.io,phiroict/docker,LuisBosquez/docker.github.io,johnstep/docker.github.io,shin-/docker.github.io,johnstep/docker.github.io,anweiss/docker.github.io,johnstep/docker.github.io,joeuo/docker.github.io,JimGalasyn/docker.github.io,joaofnfernandes/docker.github.io,docker/docker.github.io,LuisBosquez/docker.github.io,shin-/docker.github.io,londoncalling/docker.github.io,docker/docker.github.io,LuisBosquez/docker.github.io,londoncalling/docker.github.io,johnstep/docker.github.io,londoncalling/docker.github.io,shin-/docker.github.io,docker/docker.github.io,JimGalasyn/docker.github.io,joeuo/docker.github.io,joaofnfernandes/docker.github.io,anweiss/docker.github.io,LuisBosquez/docker.github.io,docker-zh/docker.github.io,joaofnfernandes/docker.github.io,joeuo/docker.github.io,shubheksha/docker.github.io,shubheksha/docker.github.io