Add a note about the methods of performance analisys

docs/Methods.md

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+# Methods
+
+There are 2 methods of measuring accurate haskell performance.
+
+## Eventlog method
+
+__Pros__:
+
+- Enables us to perform extensive analisys.
+  - We can understand the state of the system completely at any given time.
+  - We can make more elaborate windows.
+  - We can see all the other threads in a given window.
+  - The windows are not confined to a single thread.
+- Relatievely less invasive change in the RTS.
+
+__Cons__:
+
+- Overhead of measurement is more.
+- Collecting metrics while the app is running in production is not straight.
+  forward and requires more moving parts.
+
+## ThreadCPUTime primop method
+
+__Pros__:
+
+- Low measurement overhead.
+- Collecting metrics in production is straight forward.
+
+__Cons__:
+
+- Relatively more invasive.
+- Very simple analisys. Relative analisys isn't possible to understand the system.
+
+__Implementation detail__:
+
+- We need to hang the perfomance counters block from the TSO.
+- On every tield we can update the perf counters.
+- To get the updated value of the counter we need to yield.