docs: add documentation for concurrency experiment

runtime/experiments/concurrency/README.md

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+# Concurrency
+
+## Question
+
+_How does increasing levels of concurrent client requests affect tail latency, throughput, and the success/error rate of sandbox execution?_
+
+## Independent Variable
+
+- The number of concurrent client requests made at a given time
+
+## Dependent Variables
+
+- p50, p90, p99, and p100 latency measured in ms
+- throughput measures in requests/second
+- success rate, measures in % of requests that return a 200
+
+## Assumptions about test environment
+
+- You have a modern bash shell. My Linux environment shows version 4.4.20(1)-release
+- `hey` (https://github.com/rakyll/hey) is available in your PATH
+- You have compiled `sledgert` and the `empty.so` test workload
+
+## To Execute
+
+1. Run `./run.sh`
+2. View the results in the newest timestamped directory in `./res`
+
+## To Debug
+
+1. Run `./debug.sh` in a tab
+2. Run `./run.sh -d` in a second tab
+
+## TODO
+
+- Harden scripts to validate assumptions
+- Improve error handling in scripts. If `sledgrt` crashes, this charges forward until it hits a divide by error when attempting to clean data that doesn't exist