Improve contribution guide

[hsloot]

Related issues

None

Checklist

• [x] R CMD CHECK successful
• [x] Tests included
• [x] Documentation included or updated
• [x] Commit messages follow commit guidelines

Optional, but recommended:

• [x] Code passes lintr::lint_package() without errors

Description

Proposed changes:

• Add paragraph on continuous benchmarks
• Minor editorial changes

[github-actions[bot]]

Benchmark summary

File	Expression	Min time	Median time	Mem. alloc
armageddon-15.R	Armageddon	575µs (-0.79%)	594µs (-0.75%)	120KB (-41.18%)
armageddon-15.R	Arnold	1.64ms (-0.61%)	1.7ms (-0.58%)	120KB (-10.45%)
armageddon-15.R	ESM	736µs (-0.19%)	747µs (-0.72%)	120KB (-1.64%)
armageddon-15.R	ExMarkovian	893µs (-0.96%)	927µs (-0.54%)	120KB (-14.29%)
armageddon-15.R	LFM	862µs (-0.99%)	878µs (-0.78%)	120KB (-2.44%)
pareto-125.R	ExMarkovian	6.2ms (-0.32%)	6.33ms (-0.31%)	979KB (0.00%)
pareto-125.R	LFM	12.5ms (-0.48%)	12.7ms (-0.62%)	979KB (0.00%)
pareto-15.R	Arnold	1.83ms (0.55%)	1.88ms (-0.53%)	120KB (0.00%)
pareto-15.R	ExMarkovian	968µs (0.46%)	996µs (-0.40%)	120KB (0.00%)
pareto-15.R	LFM	3.63ms (0.28%)	3.76ms (-0.27%)	120KB (0.00%)
poisson-15.R	Arnold	1.9ms (0.00%)	1.94ms (0.00%)	120KB (0.00%)
poisson-15.R	ExMarkovian	1ms (-0.99%)	1.02ms (-0.97%)	120KB (0.00%)
poisson-15.R	LFM	1.18ms (0.00%)	1.2ms (-0.83%)	120KB (0.00%)