add an .npmignore file which does not exclude the build dir

Per https://docs.npmjs.com/misc/developers#keeping-files-out-of-your-package,
npm automatically uses .gitignore if there is no .npmignore, which may
have been keeping the build dir out of the package

.npmignore

@@ -0,0 +1,40 @@
+# Logs
+logs
+*.log
+npm-debug.log*
+
+# Runtime data
+pids
+*.pid
+*.seed
+
+# Directory for instrumented libs generated by jscoverage/JSCover
+lib-cov
+
+# Coverage directory used by tools like istanbul
+coverage
+
+# Grunt intermediate storage (http://gruntjs.com/creating-plugins#storing-task-files)
+.grunt
+
+# node-waf configuration
+.lock-wscript
+
+# Compiled binary addons (http://nodejs.org/api/addons.html)
+build/Release
+
+# Dependency directory
+node_modules
+
+# Optional npm cache directory
+.npm
+
+# Optional REPL history
+.node_repl_history
+
+# build
+# build  <-- we want this in the package
+build-tests
+
+# vscode
+.vscode