Add pnpm commands for migration guide

fixtures/legacy-pnpm/package.json

@@ -0,0 +1,6 @@
+{
+  "private": true,
+  "devDependencies": {
+    "size-limit": ">= 0.0.0"
+  }
+}

packages/size-limit/create-help.js

@@ -10,6 +10,9 @@ function npmCommands(pkg) {
   if (existsSync(join(pkg.path, '..', 'yarn.lock'))) {
     add = 'yarn add --dev '
     rm = 'yarn remove '
+  } else if (existsSync(join(pkg.path, '..', 'pnpm-lock.yaml'))) {
+    add = 'pnpm add --save-dev '
+    rm = 'pnpm remove '
   }
   return { add, rm }
 }

packages/size-limit/test/__snapshots__/run.test.js.snap

@@ -275,6 +275,23 @@ You need to add size-limit dependency: npm install --save-dev size-limit[3
 "
 `;
 
+exports[`run shows migration guide for pnpm users 1`] = `
+"Install Size Limit preset depends on type of the project
+
+For application, where you send JS bundle directly to users
+  pnpm add --save-dev @size-limit/preset-app
+
+For frameworks, components and big libraries
+  pnpm add --save-dev @size-limit/preset-big-lib
+
+For small (< 10 kB) libraries
+  pnpm add --save-dev @size-limit/preset-small-lib
+
+Check out docs for more complicated cases
+  https://github.com/ai/size-limit/
+"
+`;
+
 exports[`run shows migration guide for yarn users 1`] = `
 "Install Size Limit preset depends on type of the project
 

packages/size-limit/test/run.test.js

@@ -140,6 +140,10 @@ describe(`run`, () => {
     expect(await error('legacy-yarn')).toMatchSnapshot()
   })
 
+  it('shows migration guide for pnpm users', async () => {
+    expect(await error('legacy-pnpm')).toMatchSnapshot()
+  })
+
   it('shows migration guide for npm users: config 1, dep 0', async () => {
     let [process, history] = createProcess('npm-with-config-without-dev')
     await run(process)