made a readme

Author: ofloveandhate

examples/custom_numeric_type/readme.md

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+# An example of using eigenpy to extend Python using Eigen linear algebra package with custom numeric types
+
+## Building
+
+1. Make a build directory.  Move into it
+2. `cmake ../`
+3. `make`
+4. `make install`
+5. Move back up a directory
+6. `python3 examplescript.py`