feat: add enhancement template

Signed-off-by: m.nabokikh <maksim.nabokikh@flant.com>

enhancements/README.md

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+# Enhancement Proposal
+
+## Why do we need it?
+
+Enhancement Proposal (EP) is a design document providing information to the community, or describing a new feature for Dex.
+
+We intend EPs to be the primary mechanisms for proposing major new features.
+This will make it easier for community to describe, track, and look through the history of changes that a going to affect the future development of a product.
+
+## Process
+
+### Before starting
+1. Search GitHub for [previous issues](https://github.com/dexidp/dex/issues) and [EPs](https://github.com/dexidp/dex/tree/master/enhancements).
+2. If an issue does not exist, [open it](https://github.com/dexidp/dex/issues/new?assignees=&labels=&template=feature_request.yaml).
+3. Ensure that writing enhancement proposal is necessary for you change by discussing it with a community.
+
+### Writing an enhancement proposal
+
+1. Fork the repo.
+2. Copy the [`enhancements/_title-YYYY-MM-DD-#issue.md`](enhancements/_title-YYYY-MM-DD-#issue.md) template with the appropriate
+   name.
+3. Fill all sections according to hints in them. Provide as much information as you can.
+4. Submit your PR and discuss it with the Dex team.

enhancements/_title-YYYY-MM-DD-#issue.md

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+# Enhancement Proposal (EP) <issue#> - <YYYY-MM-DD> - <title>
+
+## Table of Contents
+
+- [Summary](#summary)
+- [Motivation](#motivation)
+    - [Goals/Pain](#goals)
+    - [Non-Goals](#non-goals)
+- [Proposal](#proposal)
+    - [User Experience](#user-experience)
+    - [Implementation Details/Notes/Constraints](#implementation-detailsnotesconstraints)
+    - [Risks and Mitigations](#risks-and-mitigations)
+    - [Alternatives](#alternatives)
+- [Future Improvements](#future-improvements)
+
+## Summary
+
+- Provide a one-paragraph description of the expected change here.
+
+## Context
+
+- Link to any previous issues, RFCs, discussions, or briefs.
+- Link to any ongoing or future work relevant to this change.
+
+## Motivation
+
+### Goals/Pain
+
+- List work that is assumed to be done in the scope of this enhancement.
+- Mention problems solve by this enhancement.
+
+### Non-goals
+
+- List work that is entirely out of the scope of this enhancement. Use this to define EP borders to keep work focused.
+- All planned future enhancements should be listed in one of the following blocks - Future Improvements.
+
+## Proposal
+
+### User Experience
+
+- Explain your change as if you were describing it to end-users.
+- Explain the way users are supposed to use Dex with the proposed enhancement.
+
+### Implementation Details/Notes/Constraints
+
+- Explain your change as if you were at a development team meeting (give more technical and implementation details).
+- When possible, demonstrate with pseudo-code, not text.
+- Be specific. Be opinionated. Avoid ambiguity.
+
+### Risks and Mitigations
+
+- Mention all expected risks and migrations in detail here.
+- Do not forget to mention if the proposed enhancement is a breaking change.
+
+### Alternatives
+
+- What other approaches have been considered, and why did you not choose them?
+- What happens if this enhancement will never be accepted and implemented?
+
+## Future Improvements
+
+- List any future improvements.