Add open-source warning to issue and pull request template

[jonashaag]

I'm adding this to all open-source repos. See it in action: https://github.com/Quantco/multiregex/issues/new/choose

[github-actions]

(benchmark 7899746162 / attempt 1)
Base results / Our results / Change

Model	Size	Dump Time	Load Time
sklearn rf 20M	23.4 MiB / 3.0 MiB / 7.67 x	0.01 s / 0.03 s / 2.20 x	0.02 s / 0.03 s / 1.41 x
sklearn rf 20M lzma	6.6 MiB / 2.0 MiB / 3.27 x	9.29 s / 0.70 s / 0.08 x	0.41 s / 0.15 s / 0.37 x
sklearn rf 200M	238.8 MiB / 30.8 MiB / 7.75 x	0.15 s / 0.23 s / 1.58 x	0.22 s / 0.37 s / 1.68 x
sklearn rf 200M lzma	48.0 MiB / 14.8 MiB / 3.24 x	69.66 s / 12.99 s / 0.19 x	3.13 s / 1.14 s / 0.36 x
sklearn rf 1G	1302.2 MiB / 168.0 MiB / 7.75 x	0.83 s / 1.13 s / 1.36 x	1.20 s / 1.93 s / 1.60 x
sklearn rf 1G lzma	259.6 MiB / 99.2 MiB / 2.62 x	355.71 s / 72.61 s / 0.20 x	16.04 s / 6.78 s / 0.42 x
sklearn gb 2M	2.5 MiB / 1.1 MiB / 2.16 x	0.02 s / 0.21 s / 8.96 x	0.03 s / 0.21 s / 7.37 x
sklearn gb 2M lzma	0.6 MiB / 0.2 MiB / 3.80 x	0.73 s / 0.35 s / 0.48 x	0.07 s / 0.26 s / 3.87 x
lgbm gbdt 2M	2.6 MiB / 1.0 MiB / 2.78 x	0.04 s / 0.17 s / 3.88 x	0.01 s / 0.08 s / 10.59 x
lgbm gbdt 2M lzma	0.9 MiB / 0.5 MiB / 1.90 x	0.87 s / 0.34 s / 0.39 x	0.06 s / 0.11 s / 1.97 x
lgbm gbdt 5M	5.3 MiB / 1.9 MiB / 2.81 x	0.09 s / 0.35 s / 3.79 x	0.02 s / 0.17 s / 10.90 x
lgbm gbdt 5M lzma	1.7 MiB / 0.8 MiB / 1.96 x	2.00 s / 0.71 s / 0.36 x	0.11 s / 0.22 s / 2.13 x
lgbm gbdt 20M	22.7 MiB / 7.6 MiB / 3.00 x	0.38 s / 1.42 s / 3.70 x	0.07 s / 0.72 s / 10.31 x
lgbm gbdt 20M lzma	6.3 MiB / 3.0 MiB / 2.09 x	10.90 s / 3.20 s / 0.29 x	0.41 s / 0.87 s / 2.14 x
lgbm gbdt 100M	101.1 MiB / 33.0 MiB / 3.06 x	1.72 s / 6.50 s / 3.79 x	0.35 s / 28.97 s / 83.82 x
lgbm gbdt 100M lzma	25.6 MiB / 10.6 MiB / 2.41 x	52.97 s / 16.04 s / 0.30 x	1.64 s / 3.71 s / 2.26 x
lgbm rf 10M	10.9 MiB / 3.2 MiB / 3.46 x	0.17 s / 0.41 s / 2.42 x	0.03 s / 0.27 s / 10.14 x
lgbm rf 10M lzma	0.7 MiB / 0.4 MiB / 1.86 x	1.22 s / 0.59 s / 0.48 x	0.08 s / 0.30 s / 3.68 x