doc: add readme

README.md

+# easy-powercap
+
+This repository contains a [Batsim](https://batsim.readthedocs.io/en/latest/)-compatible implementation of the EASY+powercap scheduling algorithm.
+
+## Algorithm description
+This algorithm is strongly based on EASY backfilling.
+The main addition over EASY is that a powercap must be followed during a time window.
+In other words, a constraint on the maximum power consumption value is set for the whole platform.
+This algorithm uses additional data on the power consumption of jobs to take its decisions.
+
+Please read [the algorithm implementation](./easypower.cpp) for all details.
+
+## Usage
+This repository is a [Nix flake](https://nixos.wiki/wiki/Flakes) and can be used via this interface.
+Alternatively, you can compile the algorithm dynamic library via `meson` + `ninja` with a command such as `meson setup build && ninja -C build` (dependencies are defined in [flake.nix](./flake.nix) and [flake.lock](./flake.lock)).
+
+The compiled library can be used as a Batsim EDC (external decision component) library with a command such as:
+```
+batsim -l <path/to/compiled/easypower.so> 0 <INPUT-PARAMETERS> \
+       -p <path/to/platform.xml> \
+       -w <path/to/workload.json> \
+       -e <path/to/output/directory/>
+```
+
+where `<INPUT-PARAMETERS>` is a string that contains a JSON object that must at least contain:
+```
+{
+  "idle_watts": 240.0,
+  "powercap_dynamic_watts": 323946,
+  "normal_dynamic_watts": 719880.0,
+  "powercap_end_time_seconds": 10800,
+  "job_power_estimation_field": "mean_power_estimation",
+}
+```
+- `idle_watts` is the amount of watts that a **single node** consumes when idle
+- `powercap_dynamic_watts` is the value of the powercap during the constrained time window, in watts
+- `normal_dynamic_watts` is the value of the powercap after the constrained time window, in watts
+- `powercap_end_time_seconds` is the duration of the constrained time window. the time window always starts at t=0
+- `job_power_estimation_field` is the name of the field to use as the estimation of a job's dynamic power consumption.
+  Please note that this field must be set in the workload file used.
+
+## Versions
+This has been tested with the following important dependencies:
+
+- batsim commit ee797ccebbb95410479663ee0547e752112fc83e (batprotocol branch as of 2024-04-04)
+  - SimGrid release 3.34 (commit 036c801d55e3ab07b470c79640109080fed049a1, compiled by NUR-Kapack's definition  4d8ca88fd8a0a2287ee5c023877f14d53d4854c1)
+- batprotocol commit 25bc5bbf039c18a8024c4ab326047ba56800376a
+- intervalset commit 13d8f2d7000d4dc4e3202422658b0b5d83f83679
+
+Please refer to [flake.nix](./flake.nix)/[flake.lock](./flake.lock) for a full pinning of the environment to compile the library.