Declared a minimum TypeScript version. 3.7.0

docs/_tutorials/migration_v2.md

@@ -126,3 +126,7 @@ async function noBotMessages({ message, next }) {
 ### @slack/[email protected] support schedule
 
 `@slack/[email protected]` will be deprecated on **June 30th, 2020**. We plan on continuing to implement bug fixes and will also consider back-porting new features on a case by case basis up until then. Once `@slack/[email protected]` has been deprecated, we will only implement **critical bug fixes** until the official end of life date and close non critical issues and pull requests. End of life is slated for **April 30th, 2021**. At this time, development will fully stop for `@slack/[email protected]` and all remaining open issues and pull requests will be closed. 
+
+### Minimum TypeScript Version
+
+As outlined in our [using TypeScript guide](https://slack.dev/bolt/tutorial/using-typescript), `@slack/[email protected]` requires a minimum TypeScript version of 3.7.

docs/_tutorials/using-typescript.md

@@ -0,0 +1,17 @@
+---
+title: Using TypeScript
+order: 3
+slug: using-typescript
+lang: en
+layout: tutorial
+permalink: /tutorial/using-typescript
+---
+# Using TypeScript
+
+This project is written and built using [TypeScript](https://www.typescriptlang.org/), which means many of the APIs have type information metadata 🎉. If you’re using a code editor like VSCode, Atom, or many others that know how to read that metadata, or if you’re using TypeScript in your own project, you’ll benefit from improved documentation as your write code, early detection of errors, easier refactoring, and more.
+
+This page helps describe how to use this package from a project that also uses TypeScript.
+
+### Minimum version
+
+The latest major version of `@slack/bolt` is supported to build against a minimum TypeScript version of v3.7.

package.json

@@ -69,6 +69,6 @@
     "ts-node": "^8.1.0",
     "tslint": "^5.15.0",
     "tslint-config-airbnb": "^5.11.1",
-    "typescript": "^3.4.3"
+    "typescript": "^3.7.2"
   }
 }