add citation.cff file from metadata

CITATION.cff

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+cff-version: 1.2.0
+message: "If you use this software, please cite it as below."
+authors:
+  - family-names: "Carroll"
+    given-names: "Christopher D."
+    orcid: "https://orcid.org/0000-0003-3732-9312"
+title: Optimal Financial Investment over the Life Cycle - Blog Post
+abstract: A blog post (https://econ-ark.github.io/PortfolioChoiceBlogPost/PortfolioChoiceBlogPost.html) showcasing use of HARK to the portfolio choice allocation problem.
+
+references:
+  - type: blog
+    authors:
+      - family-names: "Carroll"
+        given-names: "Christopher D."
+        orcid: "https://orcid.org/0000-0003-3732-9312"
+    title: "Optimal Financial Investment over the Life Cycle - Blog Post" # required
+
+repository-code: https://github.com/econ-ark/PortfolioChoiceBlogPost
+

REMARK.md

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+---
+# REMARK required fields
+remark-version: "1.0"
+remark-name: "PortfolioChoiceBlogPost"
+notebooks:
+  - PortfolioChoiceBlogPost.ipynb
+
+tags:
+  - REMARK
+  - Replication
+  - Notebook
+  - Blog
+---
+
+# PortfolioChoiceBlogPost
+
+A [blog post](https://econ-ark.github.io/PortfolioChoiceBlogPost/PortfolioChoiceBlogPost.html) showcasing use of HARK to the portfolio choice allocation problem.
+