[libc++] NFC: Document the Differential queries to avoid duplicating work

[ldionne]

@Fabio Here it is. Hopefully this should prevent situations like the .contains one in the future.

@griwes @wash You were once asking for how to look for open reviews, well the queries are now public!

[zoecarver]

@ldionne thank you for adding this much-needed bit of documentation.

[zoecarver]

I don't think you need to add this to the docs, but just so people know, you can also search reviews.llvm.org.

[ldionne]

In D94343#2487940, @zoecarver wrote:

@ldionne thank you for adding this much-needed bit of documentation.

Indeed. I always feel sorry for people who spend valuable time working on something, and then we have to tell them "it's already been done". It's not acceptable for an open source project IMO.

The problem is that we did *not* have these queries 30 minutes ago. I needed admin rights to add them, and had been waiting to get them. Fortunately, I got help from the right people just now. Yay!

[ldionne]

Opened a review mostly so people would know about the query. Shipping!

libcxx/docs/Contributing.rst

 .. _ContributingToLibcxx:
 
 ======================
 Contributing to libc++
 ======================
 
 .. contents::
   :local:
 
 Please read `this document <https://www.llvm.org/docs/Contributing.html>`__ on general rules to contribute to LLVM projects.
 
 Tasks and processes
 ===================
 
 This file contains notes about various tasks and processes specific to libc++.
 
+Looking for pre-existing reviews
+================================
+
+Before you start working on any feature, please take a look at the open reviews
+to avoid duplicating someone else's work. You can do that by going to the website
+where code reviews are held, `Differential <https://reviews.llvm.org/differential>`__,
+and clicking on ``Libc++ Open Reviews`` in the sidebar to the left. If you see
+that your feature is already being worked on, please consider chiming in instead
+of duplicating work!
+
 Post-Release TODO
 =================
 
 After branching for an LLVM release:
 
 1. Update ``_LIBCPP_VERSION`` in ``include/__config``
 2. Update the ``include/__libcpp_version`` file
 3. Update the version number in ``docs/conf.py``