Update README.md build steps with note to start clean after dependenc…

…y changes/pulling to a new commit

Proposing a note in the readme for those doing manual builds to run `git clean -xfd` when updating to a newer commit of the repo, particularly when updating dependencies. 

The jenkinsfile already does this, see #349 (comment) for more info. As a casual hobbyist developer who comes back to working on Theia intermittently this issue has come back to frustrate me for a while and seems to have come up in other conversations. There may be some better ways to handle this but the `git clean -xfd` approach has seemingly been more reliable.

README.md

@@ -67,6 +67,8 @@ Documentation on how to package Theia as a Desktop Product may be found [here](h
 
 For development and casual testing of the Eclipse Theia IDE, one can build it in "dev" mode. This permits building the IDE on systems with less resources, like a Raspberry Pi 4B with 4GB of RAM.
 
+NOTE: If manually building after updating dependencies or pulling to a newer commit, run `git clean -xfd` to help avoid runtime conflicts.
+
 ```sh
 # Build "dev" version of the app. Its quicker, uses less resources, 
 # but the front end app is not "minified"