Adding release notes

doc/release-notes/0.7.6.md

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+Antimony 0.7.6
+--------------
+
+This is a major release in terms of architecture:  
+All nodes are now defined as scripts, and scripts now have the power to
+define UI primitives (points and wireframes) that show up in 3D views.
+
+Files saved with `0.7.5` and earlier will be automatically upgraded to
+using script nodes (instead of hard-coded nodes).  This backwards
+compatibility will be removed in later releases.
+
+If you have a file for which automatic upgrading fails, send it in as
+a bug report for further investigation.
+
+**Features:**
+- Added `fab.ui` namespace, with hooks to create 3D view objects from scripts.
+  - `fab.ui.point` allows points with user-defined drag functions
+  - `fab.ui.wireframe` creates user-defined wireframes
+- Added `fab.color` namespace with a set of standard colors.
+- Ported all hard-coded nodes into scripts.
+- Skip early render stages if they are sufficiently fast.
+- Show error message if Save As or Export target file isn't writable.
+- New inspectors are placed in the center of a canvas if possible.
+- Automatically upgrade hard-coded nodes in older files to scripts.
+
+**Bugfixes:**
+- Build issue: `sb` directory is now created before subfolders are copied.
+- Off-by-one line highlighting error in script panes.
+- Weird namespace issue in scripts.
+- Remove white border in script UI (Linux).