Add a changes file for bug7157.

changes/bug7157

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+ o Minor features:
+   - Alter the Path Bias log messages to be more descriptive in terms
+     of reporting timeouts and other statistics.
+   - Create three levels of Path Bias log messages, as opposed to just
+     two. These are configurable via consensus as well as via torrc
+     options PathBiasNoticeRate, PathBiasWarnRate, PathBiasExtremeRate.
+     The default values are 0.70, 0.50, and 0.30 respectively.
+   - Separate the log message levels from the decision to drop guards,
+     which also is available via torrc option PathBiasDropGuards.
+     PathBiasDropGuards defaults to 0 (off).
+   - Deprecate PathBiasDisableRate in favor of PathBiasDropGuards
+     in combination with PathBiasExtremeRate.
+   - Increase the default values for PathBiasScaleThreshold and
+     PathBiasCircThreshold from 200 and 20 to 300 and 150, respectively.