Move BUILD-README.md to eclipse_plugin root dir

[maelh]

Move the following file to eclipse_plugin/
https://github.com/AmziLS/AmziProlog/blob/master/eclipse_plugin/workspace/com.amzi.prolog-update_site/BUILD-README.md

But leave a reference in /eclipse_plugin/workspace/com.amzi.prolog-update_site/ to this file.

Also merge with /eclipse_plugin/README.md

[maelh]

Instead /eclipse_plugin/README.md now points to /eclipse_plugin/workspace/com.amzi.prolog-update_site/BUILD-README.md to avoid cluttering the main readme.

Old build instructions in /eclipse_plugin/README.md were removed (as part of the merge of the two files).