add issue weight to contributing

CONTRIBUTING.md

@@ -155,6 +155,28 @@ sudo -u git -H bundle exec rake gitlab:env:info)
 
 ```
 
+### Issue weight
+
+Issue weight allows us to get an idea of the amount of work required to solve
+one or multiple issues. This makes it possible to schedule work more accurately.
+
+You are encouraged to set the weight of any issue. Following the guidelines
+below will make it easy to manage this, without unnecessary overhead.
+
+1. Set weight for any issue at the earliest possible convenience
+1. If you don't agree with a set weight, discuss with other developers until
+consensus is reached about the weight
+1. Issue weights are an abstract measurement of complexity of the issue. Do not
+relate issue weight directly to time. This is called [anchoring](https://en.wikipedia.org/wiki/Anchoring)
+and something you want to avoid.
+1. Something that has a weight of 1 (or no weight) is really small and simple.
+Something that is 9 is rewriting a large fundamental part of GitLab,
+which might lead to many hard problems to solve. Changing some text in GitLab
+is probably 1, adding a new Git Hook maybe 4 or 5, big features 7-9.
+1. If something is very large, it should probably be split up in multiple
+issues or chunks. You can simply not set the weight of a parent issue and set
+weights to children issues.
+
 ## Merge requests
 
 We welcome merge requests with fixes and improvements to GitLab code, tests,