docs: add npm-publish docs for @dfinity/agent (#36)

* [x] Add documentation an sdk releaser can use to release commits of @dfinity/agent npm package to npm registry

.npmrc

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+git-tag-version=false

docs/npm-publish.md

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+# Publishing `@dfinity/agent` to registry.npmjs.org
+
+The goal of this document is to serve as instructions for releasing a given git checkout of this repository to registry.npmjs.org.
+
+It should be
+* accurate/functional
+* not too long or hard to manage
+* as approachable as possible, so anyone can follow the instructions
+
+## Prerequisites
+
+* [`npm`](https://www.npmjs.com/get-npm) installed and available in your shell:
+    * test:
+        ```
+        command -v npm && echo 'npm found' || echo 'ERROR: npm missing
+        ```
+
+## Instructions
+
+1. Clone the agent-js-monorepo
+    ```
+    pushd `mktemp -d`
+    git clone [email protected]:dfinity-lab/agent-js.git agent-js-monorepo
+    cd agent-js-monorepo
+    ```
+
+2. npm install (via `npm ci`) the monorepo root package
+    ```
+    npm ci
+    ```
+
+2. Change directories into the `@dfinity/agent` package (the monorepo contains several npm packages)
+    ```
+    cd packages/agent
+    ```
+
+3. Set the version number to something new:
+    ```
+    new_agent_js_version_number='0.0.0-dev.0'
+    # You may use new_agent_js_version=$NEW_DFX_VERSION
+
+    npm version $new_agent_js_version
+    ```
+
+4. Publish to npm registry:
+    ```
+    # If you haven't yet, you may need to `npm login` or request permissions from @gobengo or @hansl
+    npm publish
+    ```
+
+5. Verify the new version appears at https://www.npmjs.com/package/@dfinity/agent?activeTab=versions
+
+Afterward:
+
+* You may want to invoke `popd` until you're back to the directory you started in.