DOC: contributing: explain how to set the asv reporting threshold
The default `asv continuous` reporting threshold is 100% change (2x), which probably is far too large for pandas use case, so instruct how to set it lower. (Probably the asv default should be changed, but that's a different issue.)

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13807?src=pr) is 85.23% (diff: 100%)

> Merging [#13807](https://codecov.io/gh/pydata/pandas/pull/13807?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #13807   diff @@
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  Files           140        140          
  Lines         50415      50415          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          42970      42970          
  Misses         7445       7445          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [8f64ad7...4014ebb](https://codecov.io/gh/pydata/pandas/compare/8f64ad700bb254e17fdb006b08199a2cc6ee5f36...4014ebbf7e381f28839814d2ddd0036759284b7f?src=pr)
 Yes, thanks! That's a better default
