Update readme with existing experiments (dotnet/runtimelab#8)

* Update readme with existing experiments
* Add "CreateAnExperiment.md"

Commit migrated from dotnet/runtimelab@542dd85

CreateAnExperiment.md

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+# Create an experiment
+
+Experiments should be contained within a branch in the repository. Instead of using forks of the `dotnet/runtimelab` repository to house experiments, keep branches in the official repository which helps with community visibility. Once an experiment branch is pushed up, remember to submit a PR to update the [README.MD](README.MD#Active%20Experimental%20Projects) in the [main branch][main_branch_link] with the name of the branch and a brief description of the experiment.
+
+Things to consider:
+
+- Experiments often involve updates to the [runtime](https://github.com/dotnet/runtime). Instead of branching off of the [main branch][main_branch_link] consider branching from the [official runtime branch](https://github.com/dotnet/runtimelab/tree/runtime-master). Including the entire runtime permits a self contained experiment that is easy for the community to try out.
+
+<!-- common links -->
+
+[main_branch_link]: https://github.com/dotnet/runtimelab/tree/master