stat_dev: addition of .md file for mean

Author: jvdp1

src/stdlib_experimental_stat.md

@@ -0,0 +1,43 @@
+# Descriptive statistics
+
+## Implemented
+
+ * `mean`
+
+## MEAN - mean of array elements
+
+### Description
+
+Returns the mean of all the elements of *ARRAY*, or of the elements of *ARRAY* along dimension *DIM*.
+
+### Syntax
+
+RESULT = mean(*ARRAY*)
+
+RESULT = mean(*ARRAY*, *DIM*)
+
+### Arguments
+
+*ARRAY*: Must be an array of type INTEGER, or REAL.
+
+*DIM* (optional): Must be a scalar of type INTEGER with a value in the range from 1 to n, where n equals the rank of *ARRAY*.
+
+### Return value
+
+If *ARRAY* is of type REAL, the result is of the same type as ARRAY.
+If *ARRAY* is of type INTEGER, the result is of type as *double precision*.
+
+If *DIM* is absent, a scalar with the mean of all elements in *ARRAY* is returned. Otherwise, an array of rank n-1, where n equals the rank of *ARRAY*, and a shape similar to that of *ARRAY* with dimension *DIM* dropped is returned.
+
+### Example
+
+```fortran
+program test
+    use stdlib_experimental_stat, only: mean
+    implicit none
+    real :: x(1:6) = (/ 1., 2., 3., 4., 5., 6. /)
+    print *, mean(x)                            !returns 21.
+    print *, mean( reshape(x, (/ 2, 3 /) ))     !returns 21.
+    print *, mean( reshape(x, (/ 2, 3 /) ), 1)  !returns (/ 3., 7., 11. /) 
+end program
+```