[apl/en] Add a short APL introduction (#722)

apl.apl

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+⍝ These examples can be tested here:
+⍝    http://ngn.github.io/apl/web/index.html
+⍝
+⍝ Comments in APL are prefixed by ⍝.
+
+⍝ A list of numbers. (¯ is negative)
+2 3e7 ¯4 50.3
+
+⍝ An expression, showing some functions. In APL, there's
+⍝ no order of operations: everything is parsed right-to-
+⍝ left. This is equal to 5 + (4 × (2 ÷ (5 - 3))) = 9:
+5 + 4 × 2 ÷ 5 - 3        ⍝ 9
+
+⍝ These functions work on lists, too:
+1 2 3 4 × 5              ⍝ 5 10 15 20
+1 2 3 4 × 5 6 7 8        ⍝ 5 12 21 32
+
+⍝ All functions have single-argument and dual-argument
+⍝ meanings. For example, "×" applied to two arguments
+⍝ means multiply, but when applied to only a right-hand 
+⍝ side, it returns the sign:
+
+× ¯4 ¯2 0 2 4            ⍝ ¯1 ¯1 0 1 1
+
+⍝ Values can be compared using these operators (1 means
+⍝ "true", 0 means "false"):
+
+10 20 30 = 10 20 99      ⍝ 1 1 0
+
+10 20 30 < 10 20 99      ⍝ 0 0 1
+
+⍝ "⍳n" returns a vector containing the first n naturals.
+⍝ Matrices can be constructed using ⍴ (reshape):
+4 3 ⍴ ⍳5                 ⍝ 0 1 2
+                         ⍝ 3 4 0
+                         ⍝ 1 2 3
+                         ⍝ 4 0 1
+
+⍝ Single-argument ⍴ gives you the dimensions back:
+⍴ 4 3 ⍴ ⍳5               ⍝ 4 3
+
+⍝ Values can be stored using ←. Let's calculate the mean
+⍝ value of a vector of numbers:
+A ← 10 60 55 23
+
+⍝ Sum of elements of A (/ is reduce):
++/A                      ⍝ 148
+
+⍝ Length of A:
+⍴A                       ⍝ 4
+
+⍝ Mean:
+(+/A) ÷ (⍴A)             ⍝ 37
+
+⍝ We can define this as a function using {} and ⍵:
+mean ← {(+/⍵)÷⍴⍵}
+mean A                   ⍝ 37