Update PULL_REQUEST_TEMPLATE.md

see #855

.github/PULL_REQUEST_TEMPLATE.md

@@ -4,3 +4,16 @@ Please provide package links to:
 - godoc.org: 
 - goreportcard.com: 
 - gocover.io: 
+
+
+Note, that new categories can be added only when there are 3 packages or more.
+
+Make shure that you've checked the boxes below before you submit PR:
+- [ ] I have added my package in alphabetical order
+- [ ] I know that this package was not listed before
+- [ ] I have added godoc link
+- [ ] I have added gocover.io link
+- [ ] I have added goreportcard link
+- [ ] I have read [Contribution guidelines](https://github.com/avelino/awesome-go/blob/master/CONTRIBUTING.md#contribution-guidelines) and [Quality standard](https://github.com/avelino/awesome-go/blob/master/CONTRIBUTING.md#quality-standard).
+
+Thanks for your PR, you're awesome :+1: