Added a readme

src/Tools/RepoUtil/README.md

+# RepoUtil
+
+## Usage
+
+This is a tool used to manage the inputs and outputs of our repo.  In particular, it is used to manage our use of NuGet packages.  It helps to automate the otherwise tedious process of updating NuGet references by:
+
+- Fixing up project.json files
+- Generating helper files with specified package versions. Example [Dependencies.props](https://github.com/dotnet/roslyn/blob/master/build/Targets/Dependencies.props)
+
+The tool works by using all of our project.json files as the primary source of truth for the repo.  All files with the name pattern `*project.json` are considered to be NuGet assets and will be scanned for NuGet references.  
+
+This will impose a minimum of requirements on our NuGet packages.  In particular the tool assumes that all references to a given NuGet package should occur at the same version.  If the tool sees a package being used with different versions it will issue an error.  
+
+There are valid cases where a package is used at more than one version in the repo.  Typically because it's being deployed as an asset, used as a tool, etc ...  In those cases an entry can be added to [RepoData.json](https://github.com/dotnet/roslyn/blob/master/src/Tools/RepoUtil/RepoData.json)) in the fixed table to call out that usage:
+
+``` json
+"fixed" {
+    "Microsoft.VSSDK.BuildTools": [ "14.3.25407", "15.0.25201-Dev15Preview2" ]
+}
+```
+
+## Commands
+
+This tool operates by having a set of sub commands that it executes
+
+### change
+
+The `change` command has two main purposes:
+
+1. Update a NuGet package reference to a new version 
+2. Regenerate all of the supporting files
+
+In order to change a single reference simply pass the new package as an argument:
+
+``` cmd
+RepoUtil change "System.Collections.Immutable 1.3.0"
+```
+
+For large number of packages a file can be used to list out all of the packages.  
+
+``` cmd
+RepoUtil change -version e:\path\to\file.txt
+```
+
+To regenerate all of the supporting files without updating any packages just use `change` without any arguments
+
+``` cmd
+RepoUtil change
+```
+
+### verify
+
+The `verify` command will simply analyze the state of the repo and ensure all of the NuGet dependencies are up to date:
+
+- All packages are referenced at the same version.
+- All the generated files are up to date.
+- etc ... 
+
+
+All packages must fall into one of the following categories:
+
+- Normal: Packages which are expected to be updated.  
+- Fixed: Package + version which are referenced at a version that should never change. 
+
+All uses of a normal package in the repo must have the same version.  For example every use of System.Collections.Immutable.
+
+An example of a normal package is System.Collections.Immutable.  This package changes
+
+### consumes
+
+The `consumes` command produces a json file describing all of the NuGet packages consumed by the repo.  It essentially aggregates all of the project.json files and adds a bit of metadata on top of them. 
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