Add maintainer cheatsheet (#42)

I'm doing this so rarely that I need a checklist, let's have one.

Author: ilammy

MAINTENANCE.md

@@ -0,0 +1,82 @@
+Maintainer Cheatsheet
+=====================
+
+## Making a new release
+
+1. Make sure your tree is in sync with the mothership
+
+    ```
+    git fetch
+    git checkout master && git pull
+    git checkout release/v1 && git pull
+    ```
+
+2. Merge `master` into `release/v1`
+
+    ```
+    git merge master --no-commit
+    vim package.json package-lock.json          # update "version"
+    npm ci --omit=dev
+    git add node_modules
+    git commit -p -m "setup-nasm v1.X.X"
+    ```
+
+3. Tag and sign the release
+
+    ```
+    git tag --sign v1.X.X
+    ```
+
+    Tag comment looks like this:
+    ```
+    setup-nasm v1.5.0
+
+    - Update default NASM to 2.16.01
+    ```
+
+4. Push the new tag and `release/v1` branch
+
+    ```
+    git push origin release/v1 v1.X.X
+    ```
+
+    Visit and check that it looks okay and CI passes:
+
+    * https://github.com/ilammy/setup-nasm/tree/release/v1
+    * https://github.com/ilammy/setup-nasm/tags
+
+5. Draft the release notes
+
+    Visit and prepare a draft
+
+    * https://github.com/ilammy/setup-nasm/releases/new
+
+    Release title and comment look like this
+
+    ```markdown
+    setup-nasm v1.5.0
+
+    * Update package-lock.json (https://github.com/ilammy/setup-nasm/pull/39)
+    * Update default nasm version (https://github.com/ilammy/setup-nasm/pull/38)
+
+    New contributors:
+
+    * @Brooooooklyn
+    ```
+
+    Click the `[Save draft]` button
+
+    Check how it looks in https://github.com/ilammy/setup-nasm/releases
+
+6. Publish the release
+
+    Press the `[Publish release]` button
+
+    Check how it looks in https://github.com/ilammy/setup-nasm/releases
+
+7. Push the updated `v1` tag alias
+
+    ```
+    git tag --force v1 v1.X.X
+    git push --force origin v1
+    ```