Finding inverse of a matrix using cached computation

There are two functions. The first one `makeCacheMatrix' returns a list
of
-- set(x)  (setting the matrix `x')
-- get() (get the matrix back)
-- compute_inv() (function to compute the inverse of `x')
-- getinv() (function to return the inverse)

The second function cacheSolve(x, ...) computes the inverse if it is not
in cache, or returns the inverse if it is in the cache.
Computing inverse of a matrix using cached computation

Changes: Comment section describes the functions, gives example usage.