How to model an “is a” relationship in an ERD using Crow’s foot notation?
In ER modeling, IsA relationships go by the name "generalization/specialization".  A search on this should give you a few pointers.  Diagramming this pattern requires using a notation that was originally called "Enhanced ER modeling", although it's been a long time since this extension was introduced.  

Here's the wikipedia article:

http://en.wikipedia.org/wiki/Enhanced_entity%E2%80%93relationship_model

and here are some sample diagrams, from googling "Enhanced ER Images"

https://www.google.com/search?q=enhanced+er+images&amp;rlz=1C1CHFX_enUS586US586&amp;espv=2&amp;biw=1600&amp;bih=775&amp;tbm=isch&amp;imgil=E75HgoIGspploM%253A%253B5D6WMu1kSMfWrM%253Bhttp%25253A%25252F%25252Fwww.siue.edu%25252F~dbock%25252Fcmis450%25252F4-eermodel.htm&amp;source=iu&amp;pf=m&amp;fir=E75HgoIGspploM%253A%252C5D6WMu1kSMfWrM%252C_&amp;usg=__MdUOOzgIiqsPSTBQjFkTyRuaAlQ%3D&amp;dpr=1&amp;ved=0CDEQyjc&amp;ei=TpMBVeDoKMOjgwSerIOYBg#imgdii=_

I generally use the triangle notation.