Software development effort estimation (SDEE) generally involves leveraging the information about the effort spent in developing similar software in the past. Most organizations do not have access to sufficient and reliable forms of such data from past projects. As such, the existing SDEE methods suffer from low usage and accuracy. We propose an efficient SDEE method for open source software, which provides accurate and fast effort estimates. The significant contributions of our paper are i) Novel SDEE software metrics derived from developer activity information of various software repositories, ii) SDEE dataset comprising the SDEE metrics' values derived from $\approx13,000$ GitHub repositories from 150 different software categories, iii) an effort estimation tool based on SDEE metrics and a software description similarity model. Our software description similarity model is basically a machine learning model trained using the Paragraph Vectors algorithm on the software product descriptions of GitHub repositories. Given the software description of a newly-envisioned software, our tool yields an effort estimate for developing it. Our method achieves the highest Standard Accuracy score of 87.26% (with cliff's $δ$=0.88 at 99.999% confidence level) and 42.7% with the Automatic Transformed Linear Baseline model. Our software artifacts are available at https://doi.org/10.5281/zenodo.5095723.