Create xdai_reserve

spec/xdai_reserve

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+# Title
+
+<!--
+valid status values are: Pre-draft|Draft|Proposal|Accepted
+-->
+* Authors: Firstname Lastname <email> (url)
+* Date: YYYY.MM.DD
+* Version: 1
+* Status: Pre-draft
+
+## Rationale
+<!--
+Why are you proposing this?
+-->
+
+## Before 
+<!--
+What is the current state of the described topic?
+-->
+
+## After
+<!--
+How will things be different after this has been done?
+-->
+
+## Implementation
+<!--
+Here is the description of how these changes should be implemented.
+Please use subheadings to improve readability.
+Some suggestions:
+
+### Workflow
+
+
+### Variables
+
+### Interface
+-->
+
+## Testing
+<!--
+Please describe what test vectors that are required for this implementation
+-->
+
+## Changelog
+<!--
+Please remember to describe every change to this document in the changelog using 
+serial number:
+
+* version 1:
+-->