extend 'share' for micro:bit specific info

docs/share.md

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+# @extends
+
+## Editing Shared Projects #editing
+
+By default, all shared projects in MakeCode can be copied and edited. There is no way to share a read-only project, or a project where people can’t make a copy or can’t see the code. When a user opens a project from a Share Link (either anonymous or persistent), they can see the simulator, the blocks, or the text code. They can now download the code onto the micro:bit. They may also make a copy of the project and edit it by pressing the Edit or Edit Code buttons in the top right corner. Any edits they make will not change the original project - if they want to share their changes back, they will have to create a new share link.
+
+![Edit shared project button](/static/share/edit-shared-project.png)