Create pull request template (#944)

* Update README.md

* [skip ci] Add IVFLAT links

* [skip ci] Minor

* Create PULL_REQUEST_TEMPLATE.md

.github/PULL_REQUEST_TEMPLATE.md

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+**What type of PR is this?**
+
+api-change / bug / design / documentation / feature
+
+
+**What this PR does / why we need it:**
+
+
+**Which issue(s) this PR fixes:**
+
+Fixes #
+
+
+**Special notes for your reviewer:**
+
+
+**Does this PR introduce a user-facing change?:**
+
+
+**Additional documentation (e.g. design docs, usage docs, etc.):**
+