chore: add BUILTIN.md, append CHANGELOG.md

Change-Id: I12e0c6ba3621d383fa46a811abbdcd55dbf80a17

docs/BUILTIN.md

@@ -0,0 +1,44 @@
+# Builtin
+
+## Assertion Methods
+
+### Usage
+In "teststeps" of each json/yaml testcase, the "validate" part contains four fields: "check", "assert", "expect" and 
+"msg", when using assertion methods, method name should be put in "assert" field. The assertion result of "check" 
+element will be checked out using the regulation you put in "assert" field and compared with the element in "expect"
+field.
+
+### Method List
+
+- equals: assert the element to check equals the expected element.
+- equal: alias for equals.
+- greater_than: assert the element to check is greater than the expected element.
+- less_than: assert the element to check is less than the expected element.
+- greater_or_equals: assert the element to check is greater than or equal with the expected element.
+- less_or_equals: assert the element to check is less than or equal with the expected element.
+- not_equal: assert the element to check is not equal with the expected element.
+- contained_by: assert the expected element contains the element to check.
+- regex_match: assert the element to check matches the expected element using regex.
+- type_match: assert the element to check matches the expected element in type.
+- startswith: assert the element to check starts with the expected element.
+- endswith: assert the element to check ends with the expected element.
+- length_equals: assert the length of the element to check is equal with the expected element.
+- length_equal: alias for length_equals.
+- contains: assert the element to check contains the expected element.
+- string_equals: assert the string is equal with the expected string.
+
+## Common Functions
+
+### Usage
+The common functions are useful during the variables configuration, you can use "${FUNCTION_NAME}" to call the specific 
+function to define variables.
+
+### Function List
+- get_timestamp: get the thirteen-digit timestamp of current time. (call without argument)
+- sleep: sleep n seconds to simulate the thinking time. (call with one argument n)
+- gen_random_string: get the n-digit random string. (call with one argument n)
+- max: get the maximum of two numbers m and n. (call with two argument m and n)
+- md5: get the MD5 of the input string s. (call with one argument s)
+
+
+

docs/CHANGELOG.md

@@ -1,5 +1,7 @@
 # Release History
 
+-feat: add more plentiful response assertion methods
+
 ## v0.3.0 (2021-12-22)
 
 - feat: implement `transaction` mechanism for load test

internal/builtin/function.go

@@ -13,7 +13,7 @@ var Functions = map[string]interface{}{
 	"sleep":             sleep,           // call with one argument
 	"gen_random_string": genRandomString, // call with one argument
 	"max":               math.Max,        // call with two arguments
-	"md5":               MD5,
+	"md5":               MD5,			  // call with one argument
 }
 
 func init() {