Adds installationMethod key to package.json

requested upstream yarnpkg/yarn#1326 (comment)

testing/yarn/add-apk-install-method.patch

@@ -0,0 +1,48 @@
+Only in dist: .DS_Store
+diff -urp dist/lib/cli/commands/install.js patched/lib/cli/commands/install.js
+--- dist/lib/cli/commands/install.js	2017-02-01 00:36:23.000000000 +0000
++++ patched/lib/cli/commands/install.js	2017-02-01 01:25:41.000000000 +0000
+@@ -226,6 +226,10 @@ function getUpdateCommand() {
+     return 'choco upgrade yarn';
+   }
+
++  if (YARN_INSTALL_METHOD === 'apk') {
++    return 'apk update && apk add -u yarn';
++  }
++
+   return null;
+ }
+
+diff -urp dist/lib-legacy/cli/commands/install.js patched/lib-legacy/cli/commands/install.js
+--- dist/lib-legacy/cli/commands/install.js	2017-01-30 14:13:43.000000000 +0000
++++ patched/lib-legacy/cli/commands/install.js	2017-02-01 01:32:32.000000000 +0000
+@@ -238,6 +238,10 @@ function getUpdateCommand() {
+     return 'choco upgrade yarn';
+   }
+
++  if (YARN_INSTALL_METHOD === 'apk') {
++    return 'apk update && apk add -u yarn';
++  }
++
+   return null;
+ }
+
+@@ -1011,4 +1015,4 @@ function setFlags(commander) {
+   commander.option('-O, --save-optional', 'DEPRECATED - save package to your `optionalDependencies`');
+   commander.option('-E, --save-exact', 'DEPRECATED');
+   commander.option('-T, --save-tilde', 'DEPRECATED');
+-}
+\ No newline at end of file
++}
+Only in dist/node_modules: .DS_Store
+diff -urp dist/package.json patched/package.json
+--- dist/package.json	2017-02-01 01:23:37.000000000 +0000
++++ patched/package.json	2017-02-01 01:23:50.000000000 +0000
+@@ -1,6 +1,6 @@
+ {
+   "name": "yarn",
+-  "installationMethod": "tar",
++  "installationMethod": "apk",
+   "version": "0.20.0",
+   "license": "BSD-2-Clause",
+   "preferGlobal": true,