update readme to reflect optional nature of numpy

README.md

@@ -113,11 +113,15 @@ If you'd like to help out financially, Aspidites' maintainer accepts donations.
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 ## Core Dependencies
-Aspidites has just 3 core dependencies, all licensed under a compatible OSI approved license. In general, dependencies are vendored unless they are commonly-used libraries. 
+Aspidites has just 3 core dependencies, two required and one optional, one all licensed under a compatible OSI approved license. In general, dependencies are vendored unless they are commonly-used libraries. 
+
+Required:
 - cython
-- numpy
 - future
 
+Optional:
+- numpy
+
 In addition to the core dependencies are the 4 optional dependencies, all licensed under a compatible OSI approved license, needed to run the canned test-suite.
 - pytest
 - pytest-xdist