loadtest: add README

Author: GeorgeTsagk

itest/loadtest/README.md

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+## Description
+
+This directory (`itest/loadtest`) includes all files and data related to running
+the loadtesting suite for taproot assets daemon. These tests use the existing
+itest framework to run against real external running daemons.
+
+The configuration file needs to be named `loadtest.conf` and must be placed on
+the working directory in order for the loadtest executable to detect it. A
+sample configuration can be found in `loadtest-sample.conf` which includes all
+the fields that are required for the tests to run successfully. This includes
+connection credentials for the tapd & lnd nodes, as well as a bitcoind backend.
+
+For further tracking and metrics, a prometheus gateway is configured and used by
+the loadtests in order to submit any desired data in-flight.
+
+## Building
+
+To create the loadtest executable run `make build-loadtest`. This will
+create a `loadtest` binary in your working directory which you can run, given
+that you have a correct `loadtest.conf` in the same directory.
+
+The executable will consult the appropriate fields of `loadtest.conf` and it's
+going to run the defined test case with the respective config.
+
+Example: To run a mint loadtest which mints batches of `450` assets we will
+define `test-case="mint"` and `mint-test-batch-size=450` in our `loadtest.conf`.
+
+## Using dev-resources docker setup
+
+You can use any kind of external running daemon, as long as it's reachable. The
+easiest way to spin up some nodes from scratch for the purpose of the loadtests
+is to run the `dev-resources/docker-regtest` setup and use `alice`, 
+`alice-tapd`, `bob`, `bob-tapd` and the single `bitcoind` instance.