Add pipools/_vendored/README.rst

piptools/_vendored/README.rst

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+Vendored libs policy
+====================
+
+* Vendored libraries **MUST** not be modified except as required to
+  successfully vendor them.
+
+* Vendored libraries **MUST** be released copies of libraries available on
+  PyPI.
+
+* The versions of libraries vendored in pip-tools **MUST** be reflected
+  in the section below.
+
+* Vendored libraries **MUST** function without any build steps such as 2to3 or
+  compilation of C code, practically this limits to single source 2.x/3.x and
+  pure Python.
+
+* Any modifications made to libraries **MUST** be noted in the section below.
+
+
+Versions and modifications:
+===========================
+
+* ``pip`` == 9.0.3
+
+  Modifications:
+    - None.
+
+  Reason: ``pip`` 10.x internal changes break the dependency resolution code for ``pip-compile``.
+    Note that ``pip-sync`` still uses the user-installed ``pip`` to perform the install/uninstall operations.