Update docs for docs.ohif.org

docs/I-want-to/add-a-tool-to-the-viewer.md

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+# Add a Tool to the Viewer
+
+To add a tool to the Viewer there are a few steps:
+
+1. Add the tool itself to the repository.
+
+  If you're using something from Cornerstone Tools you can skip this step.
+
+  Some examples of custom tools can be found in the lesion tracker:   https://github.com/OHIF/Viewers/tree/master/Packages/ohif-lesiontracker/client/compatibility
+
+2. Add the toolbar button itself to the array of tools in the Toolbar:
+  https://github.com/OHIF/Viewers/blob/574a6d02b090b8b2f020430c5919f8377b8316c6/OHIFViewer/client/components/toolbarSection/toolbarSection.js
+
+3. **A:** Add it to the toolManager (if it's a tool, such as length / angle):
+
+  https://github.com/OHIF/Viewers/blob/574a6d02b090b8b2f020430c5919f8377b8316c6/Packages/lesiontracker/client/tools.js#L2
+
+  ** --- OR --- **
+
+  **B:** Add it to the functionList if it's a command (e.g. toggle CINE play, or Invert the current viewport):
+
+  https://github.com/OHIF/Viewers/blob/574a6d02b090b8b2f020430c5919f8377b8316c6/OHIFViewer/client/components/viewer/viewer.js#L12