add design proposal template

docs/design_proposals/design-proposal-template.md

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+# Title
+
+* Author(s): \<your name\>
+* Reviewers: \<reviewer name\>
+
+    If you are already working with someone mention their name.
+    If not, please leave this empty, some one from the core team with assign it to themselves.
+* Date: \<date\>
+* Status: [Reviewed/Cancelled/Under implementation/Complete]
+
+Here is a brief explanation of the Statuses
+
+1. Reviewed: The proposal PR has been accepted, merged and ready for
+   implementation.
+2. Under implementation: An accepted proposal is being implemented by actual work.
+   Note: The design might change in this phase based on issues during
+   implementation.
+3. Cancelled: During or before implementation the proposal was cancelled.
+   It could be due to:
+   * other features added which made the current design proposal obsolete.
+   * No longer a priority.
+4. Complete: This feature/change is implemented.
+
+## Background
+
+In this section, please mention and describe the new feature, redesign
+or refactor.
+
+Please provide a brief explanation for the following questions:
+
+1. Why is this required?
+2. If this is a redesign, what are the drawbacks of the current implementation?
+3. Is there any another workaround, and if so, what are its drawbacks?
+4. Mention related issues, if there are any.
+
+Here is an example snippet for an enhancement:
+
+___
+Currently, Kaniko includes `build-args` when calculating layer cache key even if they are not used
+in the corresponding dockerfile command.
+
+This causes a 100% cache miss rate even if the layer contents are same. 
+Change layer caching to include `build-args` in cache key computation only if they are used in command.
+___
+
+## Design
+
+Please describe your solution. Please list any:
+
+* new command line flags
+* interface changes
+* design assumptions
+
+### Open Issues/Questions
+
+Please list any open questions here in the following format:
+
+**\<Question\>**
+
+Resolution: Please list the resolution if resolved during the design process or
+specify __Not Yet Resolved__
+
+## Implementation plan
+As a team, we've noticed that larger PRs can go unreviewed for long periods of
+time. Small incremental changes get reviewed faster and are also easier for
+reviewers.
+___
+
+
+## Integration test plan
+
+Please describe what new test cases you are going to consider.