Added directions on clearing compile cache

doc/doc/sandbox.txt

@@ -163,6 +163,8 @@ The compile cache is based upon the C++ code of the graph to be compiled.
 So, if you change compilation environment variables, such as
 ``THEANO_BLAS_LDFLAGS``, you will need to manually remove your compile cache.
 
+``Theano/bin/theano-compiledir clear``
+
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