Add if statement

examples/conditional_statements/if_statement.md

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+# If statement
+
+An `if` statement is a programming conditional statement that, if proved true, performs a function or displays information. Below is a general example of an if statement in V:
+
+```v
+john_height := 100
+maria_height := 178
+
+if john_height < maria_height {
+	println('$john_height < $maria_height')
+} else {
+  println('$john_height > $maria_height')
+}
+```
+
+The `if...else` statement executes two different codes depending upon whether the test expression is `true` or `false`. Sometimes, a choice has to be made from more than 2 possibilities. The `if...else if...else` ladder allows you to check between multiple test expressions and execute different statements.
+
+```v
+tom_age := 20
+ashia_age := 38
+
+if tom_age < ashia_age {
+	println('$tom_age < $ashia_age')
+} else if tom_age > ashia_age {
+	println('$tom_age > $ashia_age')
+} else {
+	println('$tom_age == $ashia_age')
+}
+```
+
+It is possible to include an `if...else` statement inside the body of another `if...else` statement.
+
+```v
+tom_age := 20
+ashia_age := 38
+
+if tom_age < ashia_age {
+  if tom_age < 18 {
+	   println('tom_age < 18 and younger than Ashia.')
+  } else {
+    println('tom_age > 18 and older than Ashia.')
+  }
+} else {
+	println('$tom_age == $ashia_age')
+  println('Tom and Ashia have the same age.')
+}
+```
+
+There are no parentheses surrounding the condition, and the braces are always required.
+
+`if` can also be used as an expression:
+
+```v
+tom_age := 20
+ashia_age := 38
+
+s := if tom_age > ashia_age {
+	'Tom is the youngest'
+}
+else {
+	'Ashia is the youngest'
+}
+print(s) // 'Tom is the youngest'
+```