doc update (#703)

CONTRIBUTING.md

-## Contributing to CoreDNS
+# Contributing to CoreDNS
 
 Welcome! Our community focuses on helping others and making CoreDNS the best it
 can be. We gladly accept contributions and encourage you to get involved!
 
-### Bug reports
+## Bug reports
 
 First, please [search this repository](https://github.com/coredns/coredns/search?q=&type=Issues&utf8=%E2%9C%93)
 with a variety of keywords to ensure your bug is not already reported.
@@ -21,14 +21,14 @@ Please be kind. :smile: Remember that CoreDNS comes at no cost to you, and you'r
 getting free help.
 
 
-### Minor improvements and new tests
+## Minor improvements and new tests
 
 Submit [pull requests](https://github.com/coredns/coredns/pulls) at any time. Make
 sure to write tests to assert your change is working properly and is thoroughly
 covered.
 
 
-### Proposals, suggestions, ideas, new features
+## Proposals, suggestions, ideas, new features
 
 First, please [search](https://github.com/coredns/coredns/search?q=&type=Issues&utf8=%E2%9C%93)
 with a variety of keywords to ensure your suggestion/proposal is new.
@@ -48,16 +48,11 @@ If you are going to spend significant time implementing code for a pull request,
 best to open an issue first and "claim" it and get feedback before you invest
 a lot of time.
 
-
-### Vulnerabilities
+## Vulnerabilities
 
 If you've found a vulnerability that is serious, please email me: <miek@miek.nl>.
 If it's not a big deal, a pull request will probably be faster.
 
-## Thank you
-
-Thanks for your help! CoreDNS would not be what it is today without your contributions.
-
 ## Git Hook
 
 We use `golint` and `go vet` as tools to warn use about things (noted golint is obnoxious sometimes,
@@ -112,3 +107,7 @@ $ dep prune
 ```
 
 Please refer to Golang's [`dep`](https://github.com/golang/dep) for more details.
+
+# Thank you
+
+Thanks for your help! CoreDNS would not be what it is today without your contributions.

middleware.md

@@ -33,7 +33,7 @@ something has been written to the client (by the middleware).
 
 See a couple of blog posts on how to write and add middleware to CoreDNS:
 
-* <https://blog.coredns.io/#> TO BE PUBLISHED.
+* <https://blog.coredns.io/2017/03/01/how-to-add-middleware-to-coredns/>
 * <https://blog.coredns.io/2016/12/19/writing-middleware-for-coredns/>, slightly older, but useful.
 
 ## Metrics
@@ -132,3 +132,15 @@ answer for the MX query:
 % dig +nocmd @localhost +noall +ans MX compute.internal
 compute.internal.	3600	IN	MX	10 mx.compute.internal.
 ~~~
+
+## Qualifying for main repo
+
+Middleware for CoreDNS can live out-of-tree, `middleware.cfg` defaults to CoreDNS' repo but other
+repos work just as well. So when do we consider the inclusion of a new middleware in the main repo?
+
+* First, the middleware should be useful for other people. "Useful" is a subjective term. We will
+  probably need to further refine this.
+* Current internet standards need be supported: IPv4 and IPv6, so A and AAAA records should be
+  handled (if your middleware is in the business of dealing with address records that is).
+* It must have tests.
+* It must have a README.md for documentation.