Updated readme with instructions; .gitignore to exclude custom extension manifest
Updated readme with instructions; .gitignore to exclude custom extension manifest

srdjan-bozovic/vso-extension-samples,Microsoft/vsts-extension-samples,Microsoft/vso-extension-samples,srdjan-bozovic/vso-extension-samples,Microsoft/vso-extension-samples,srdjan-bozovic/vso-extension-samples,Microsoft/vso-extension-samples,Microsoft/vsts-extension-samples,Microsoft/vsts-extension-samples