Add Blocks section to Tutorials

docs/SUMMARY.md

@@ -143,6 +143,7 @@
         * [Strings](tutorials/basics/40_strings.md)
         * [Control Flow](tutorials/basics/50_control_flow.md)
         * [Methods](tutorials/basics/60_methods.md)
+        * [Blocks](tutorials/basics/70_blocks.md)
 * [Manuals](man/README.md)
     * [Using the Compiler](man/crystal/README.md)
     * [The Shards Command](man/shards/README.md)

docs/tutorials/basics/60_methods.md

@@ -163,3 +163,32 @@ puts life_universe_and_everything + 1 # Error: method top-level life_universe_an
 ```
 
 Now the compiler can show us exactly where the problem is originated. As we can see, providing type information is really useful for finding errors at compile time.
+
+## Blocks
+
+While working with *collections* (just as an example) we may have came across code that looks like this:
+
+```crystal-play
+["Hello", "World", 42].each do |elem|
+  puts elem
+end
+```
+
+or maybe something like this:
+
+```crystal-play
+arr = [1, 2, 3].map do |elem|
+  elem * 2
+end
+
+puts arr
+```
+
+In the first example we are using the method [Indexable#each](https://crystal-lang.org/api/Indexable.html#each%28%26%3AT-%3E%29-instance-method).
+And in the second example we are using the method [Enumerable#map](https://crystal-lang.org/api/Enumerable.html#map%28%26%3AT-%3EU%29%3AArray%28U%29forallU-instance-method).
+
+To both methods we are passing a *block of code* (delimited by the keywords `do` and `end`) as an argument.
+
+This *block of code*, commonly called *block* is a sequence of interactions, which will be executed at some point. When exactly? It depends on the method that receives the *block* as a parameter (in these examples `each` and `map`).
+
+*Blocks* is a really interesting and important topic, so it has its own section. Let's continue [there](70_blocks.md)!