Added XML decoder (#1044)

pkg/yqlib/doc/operators/headers/string-operators.md

@@ -0,0 +1,44 @@
+# String Operators
+
+## RegEx
+This uses golangs native regex functions under the hood - See https://github.com/google/re2/wiki/Syntax for the supported syntax.
+
+
+## String blocks, bash and newlines
+Bash is notorious for chomping on precious trailing newline characters, making it tricky to set strings with newlines properly. In particular, the `$( exp )` _will trim trailing newlines_.
+
+For instance to get this yaml:
+
+```
+a: |
+  cat
+```
+
+Using `$( exp )` wont work, as it will trim the trailing new line.
+
+```
+m=$(echo "cat\n") yq e -n '.a = strenv(m)'
+a: cat
+```
+
+However, using printf works:
+```
+printf -v m "cat\n" ; m="$m" yq e -n '.a = strenv(m)'
+a: |
+  cat
+```
+
+As well as having multiline expressions:
+```
+m="cat
+"  yq e -n '.a = strenv(m)'
+a: |
+  cat
+```
+
+Similarly, if you're trying to set the content from a file, and want a trailing new line:
+
+```
+IFS= read -rd '' output < <(cat my_file)
+output=$output ./yq e '.data.values = strenv(output)' first.yml
+```