Add vendor folder to git

vendor/github.com/prometheus/client_model/README.md

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+# Background
+Under most circumstances, manually downloading this repository should never
+be required.
+
+# Prerequisites
+# Base
+* [Google Protocol Buffers](https://developers.google.com/protocol-buffers)
+
+## Java
+* [Apache Maven](http://maven.apache.org)
+* [Prometheus Maven Repository](https://github.com/prometheus/io.prometheus-maven-repository) checked out into ../io.prometheus-maven-repository
+
+## Go
+*  [Go](http://golang.org)
+*  [goprotobuf](https://code.google.com/p/goprotobuf)
+
+## Ruby
+*  [Ruby](https://www.ruby-lang.org)
+*  [bundler](https://rubygems.org/gems/bundler)
+
+# Building
+    $ make
+
+# Getting Started
+  * The Go source code is periodically indexed: [Go Protocol Buffer Model](http://godoc.org/github.com/prometheus/client_model/go).
+  * All of the core developers are accessible via the [Prometheus Developers Mailinglist](https://groups.google.com/forum/?fromgroups#!forum/prometheus-developers).