Create and update keywords.txt file

Analyze the contents of Basic_Heuristics_CS846.docx file and add new entries to the keywords.txt file. The sample is as follows:

missing, empty, invalid: invalid inactivity, long time: stale x1, x2, x3,...,xn : no-reason x1, x2, x3,...,xn : chaotic x1, x2, x3,...,xn : duplicate x1, x2, x3,...,xn : merge-conflict x1, x2, x3,...,xn : not-pr x1, x2, x3,...,xn : low-quality x1, x2, x3,...,xn : unnecessary x1, x2, x3,...,xn : replaced