Update installation instructions for Unix users who install Nim.

readme.markdown

@@ -47,6 +47,7 @@ the following commands to clone nimble, compile it and then install it.
 
     git clone https://github.com/nim-lang/nimble.git
     cd nimble
+    git clone -b v0.13.0 --depth 1 https://github.com/nim-lang/nim vendor/nim
     nim c -r src/nimble install
 
 After these steps are completed successfully, nimble will be installed

src/nimble.nim.cfg

@@ -1,3 +1,4 @@
 --path:"$lib/packages/docutils"
 --noNimblePath
---path:"$nim/"
+--path:"$nim/"
+--path:"./vendor/nim"