Add benchmark template

.github/ISSUE_TEMPLATE/benchmark-algorithm-template.md

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+---
+name: Benchmark Algorithm Template
+about: Template used for creating benchmark issues.
+title: "[BENCH]"
+labels: help wanted, performance
+assignees: ''
+
+---
+
+# Benchmark [ALGORITHM NAME]
+The goal of this issues is to add a benchmark for the `[ALGORITHM NAME]` algorithm. Benchmarks are vital to our library and allow us to measure the impact of future performance improvements.
+
+We use the [Google benchmark](https://github.com/google/benchmark/blob/main/docs/user_guide.md) framework. For inspiration, please take a look at the existing benchmarks in `/perf`.
+
+The benchmark should be added under `/perf` in a directory which resembles the file structure of the original algorithm. i.e. if the algorithm is implemented in `include/graaflib/algorithm/coloring/greedy_graph_coloring.h` then the benchmark should be added to `perf/graaflib/algorithm/coloring/greedy_graph_coloring_benchmark.cpp`.
+
+The benchmark should measure the runtime performance of the algorithm for increasing input sizes.
+
+## Running Benchmarks
+If you IDE has the necessary integrations for it, all benchmarks can be run in the IDE from the `perf/graaflib/benchmark.cpp` file.
+
+Otherwise, we can run the benchmarks from the command line:
+```bash
+# run all benchmarks
+cd build/perf && ./Graaf_perf
+```
+
+To run an individual benchmark:
+```bash
+./Graaf_perf --benchmark_filter=YOUR_BENCHMARK_NAME
+```
+
+For more options, pass the `--help` flag.
+
+
+## Definition of Done
+- [ ] A benchmark is added for the algorithm
+- [ ] Benchmark results (copy past of the output is fine) is added to the PR