add README

Signed-off-by: Jakub Sokołowski <jakub@status.im>

README.md

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+# Description
+
+This Python script generates graphs visualizing peer retention in Status app.
+
+# Details
+
+The script queries an ElasticSearch endpoint for `logstash-*` indices and aggregates counts of instances of log messages with set `peer_id` field.
+
+This data is then analyzed using [Pandas](https://pandas.pydata.org/) and graphed using [Seaborn](https://seaborn.pydata.org/) in the form of 
+
+This was built from a combination of a [CSV generating script](https://github.com/status-im/infra-utils/blob/d1e0426cc28a1b20a182f341c55c4e295912fe35/elasticsearch/unique_count.py) and a [cohort analysis](https://colab.research.google.com/drive/1Osl3EXbKR2_iwAhOHmAiCum9F6IquRX2) done by [@jakubgs](https://github.com/jakubgs) and [@bgits](https://github.com/bgits).
+
+# Example
+
+![](examples/weekly_cohorts.png)