Add Acknowledgements to README.md

[ymd-stella]

See #37.

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Track time and Relative pose error

dataset	track_time (mean)	track_time (median)	track_time (max)	RPE (mean)	RPE (median)	RPE (max)	RPE (rmse)	RPE (sse)	RPE (std)
EuRoC MH_04 (SLAM, perspective, mono)	0.021	0.021	0.055	0.015	0.011	0.291	0.022	0.319	0.016
EuRoC MH_04 (SLAM, perspective, stereo)	0.063	0.065	0.165	0.009	0.007	0.117	0.013	0.167	0.010
EuRoC MH_04 (Localization, perspective, mono)	0.017	0.017	0.033	0.011	0.009	0.211	0.016	0.174	0.012
openvslam_test_dataset 1 (SLAM, equirectangular, mono)	0.058	0.057	0.173	0.047	0.022	2.019	0.091	29.724	0.078
openvslam_test_dataset 2 (SLAM, equirectangular, mono)	0.064	0.063	0.126	0.033	0.020	0.552	0.056	10.996	0.045
openvslam_test_dataset 3 (SLAM, equirectangular, mono)	0.062	0.060	0.209	0.115	0.047	2.002	0.230	185.302	0.200