Issue #26: Add pgv_set() and pgv_get() arguments description

Author: za-arthur

Diff for: README.md

@@ -102,6 +102,28 @@ Function | Returns
 `pgv_set(package text, name text, value anyarray, is_transactional bool default false)` | `void`
 `pgv_get(package text, name text, var_type anyarray, strict bool default true)` | `anyarray`
 
+`pgv_set` arguments:
+- `package` - name of the package, it will be created if it doesn't exist.
+- `name` - name of the variable, it will be created if it doesn't exist.
+`pgv_set` fails if the variable already exists and its transactionality doesn't
+match `is_transactional` argument.
+- `value` - new value for the variable. `pgv_set` fails if the variable already
+exists and its type doesn't match new value's type.
+- `is_transactional` - transactionality of the newly created variable, by
+default it is false.
+
+`pgv_get` arguments:
+- `package` - name of the existing package. If the package doesn't exist result
+depends on `strict` argument: if it is false then `pgv_get` returns NULL
+otherwise it fails.
+- `name` - name of the the existing variable. If the variable doesn't exist
+result depends on `strict` argument: if it is false then `pgv_get` returns NULL
+otherwise it fails.
+- `var_type` - type of the existing variable. It is necessary to pass it to get
+correct return type.
+- `strict` - pass false if `pgv_get` shouldn't raise an error if a variable or a
+package didn't created before, by default it is true.
+
 ## **Deprecated** scalar variables functions
 
 ### Integer variables