Readme should include install command line for preview build

[dzmitry-lahoda]

Cannot install template using readme.md copy paste:

F:\src\dotnet\fsharp-examples\src\benchmarks>dotnet new -i "TheAngryByrd.BenchmarkDotNet.Templates::*"
  Restoring packages for C:\Users\dzmitry\.templateengine\dotnetcli\v3.0.100-preview-009812\scratch\restore.csproj...
C:\Users\dzmitry\.templateengine\dotnetcli\v3.0.100-preview-009812\scratch\restore.csproj : error NU1103: Unable to find a stable package TheAngryByrd.BenchmarkDotNet.Templates with version
C:\Users\dzmitry\.templateengine\dotnetcli\v3.0.100-preview-009812\scratch\restore.csproj : error NU1103:   - Found 2 version(s) in nuget.org [ Nearest version: 0.1.0-alpha002 ]
C:\Users\dzmitry\.templateengine\dotnetcli\v3.0.100-preview-009812\scratch\restore.csproj : error NU1103:   - Found 0 version(s) in Microsoft Visual Studio Offline Packages
C:\Users\dzmitry\.templateengine\dotnetcli\v3.0.100-preview-009812\scratch\restore.csproj : error NU1103:   - Found 0 version(s) in C:\Program Files\dotnet\sdk\NuGetFallbackFolder
C:\Users\dzmitry\.templateengine\dotnetcli\v3.0.100-preview-009812\scratch\restore.csproj : error NU1103:   - Found 0 version(s) in https://dotnetfeed.blob.core.windows.net/dotnet-core/index.json
  Generating MSBuild file C:\Users\dzmitry\.templateengine\dotnetcli\v3.0.100-preview-009812\scratch\obj\restore.csproj.nuget.g.props.
  Generating MSBuild file C:\Users\dzmitry\.templateengine\dotnetcli\v3.0.100-preview-009812\scratch\obj\restore.csproj.nuget.g.targets.
  Restore failed in 1 sec for C:\Users\dzmitry\.templateengine\dotnetcli\v3.0.100-preview-009812\scratch\restore.csproj.

[TheAngryByrd]

Looks like it's an issue with it being a prerelease. You can insert the exact version

dotnet new -i "TheAngryByrd.BenchmarkDotNet.Templates::0.1.0-alpha002"

I'm publishing 0.1.0 right now so it should work whenever nuget indexing catches up.