Fix broken link in Readme.md (#138)

* Fix broken link in Readme.md

The linked document for how to write a policy not longer exists. This replacement link seems like it might cover similar material (but I'm guessing, because I don't know the content that was at the now-broken link).

* update broken link

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Co-authored-by: andreatp <[email protected]>

Author: tombentley

Readme.md

@@ -135,7 +135,7 @@ At the moment the following builtins are supported(and, by default, automaticall
 ### Writing the policy
 
 See
-[https://www.openpolicyagent.org/docs/latest/how-do-i-write-policies/](https://www.openpolicyagent.org/docs/latest/how-do-i-write-policies/)
+[https://www.openpolicyagent.org/docs/policy-language](https://www.openpolicyagent.org/docs/policy-language)
 
 ### Compiling the policy