Merge pull request soutaro#1353 from tk0miya/docs/steep_ignore

docs: Add document for steep:ignore comment

manual/ignore.md

@@ -0,0 +1,20 @@
+# Ignore diagnostics
+
+Steep allows you to ignore diagnostics by adding comments to your code.
+
+```ruby
+# Ignoring a range of lines
+
+# steep:ignore:start
+
+foo()      # NoMethod is detected, but ignored
+
+# steep:ignore:end
+```
+
+```ruby
+# Ignoring a specific line
+
+foo() # steep:ignore
+foo() # steep:ignore NoMethod
+```