Updated of README.md after following and implementing the previous instructions.

Was already merged in Bitbucket before import, marked as merged by the import user

I followed the instructions I wrote yesterday, from downloading a vanilla Eclipse package up to building an update site. I fixed some instructions, worded a few things differently, added a section about debugging the source code, pushed the Troubleshooting section to the end of the file and added a table of contents. I'll let native English speakers fix any errors. I deem the document will be accurate for some time in the future.