Fixed lua version errors

README.md

@@ -2,13 +2,15 @@
 
 This tool helps to download all images in markdown document and put them into one folder with changed paths
 
+**Warning**: lua 5.2 version is required
+
 ## Dependency installation:
 
 ```bash
 luarocks install luasec
 ```
 
-## Usage: 
+## Usage:
 
 ```bash
 md-parser [-o <output>] [-s <server>] [-c <config>] [-u] [-h]

src/process_md.lua

@@ -4,7 +4,7 @@ function process_md.get_file_content(path)
     local f, err = io.open(path, "r+")
     assert(f, err)
 
-    local content = f:read("a")
+    local content = f:read("*a")
 
     f:close()
     return content