[docs] Add PR template and clarify requirements to the PR description.

PULL_REQUEST.md

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+# Title
+
+The title of the PR should be short and expressive enough to reflect the proposed changes.
+It should start with the name of the subsystem affected by the PR in the square braces.
+For example:
+* [quantization] Properly quantize ReLU
+* [opencl] Define a type cl_host_size_t exactly matching the type size_t used on the host side
+* [Type.h] Add comments for the ElemKinds
+
+Ideally, the same format should be used in the commit message.
+
+# Description
+
+The description field should include a more verbose explanation of what this PR does.
+If this PR causes a change in behavior it should document the behavior before and after.
+If it fixes a bug, please, describe what the original issue is and how the change resolves it.
+
+# Formatting
+
+Please, run clang-format before submitting the PR.
+
+# Testing
+
+The testing section should include an explanation of what testing was done.
+For example unit test, manual testing through stand-alone testing binaries like mnist, cifar10, etc.
+Please use your best judgment or ask for guidance if you are unsure what kind of testing is required for a given change.
+The riskier the change, the more comprehensive the testing should be.
+If PR fixes an existing bug, make sure to present unit test which reveals the original problem.
+
+# Documentation
+
+Please, make sure to update existing docs if your change affects those.
+Docs can be found under the glow/docs. Use your best judgment for creating new docs.
+
+# Issues
+
+If the PR fixes an existing issue, please add a line of the form:
+* Fixes #Issue.
+
+This way Github will make sure that underlying referenced issue will be automatically closed once PR is merged.
+
+If the purpose of the PR to address issue partially, please, just tag the issue:
+* #Issue.
+

PULL_REQUEST_TEMPLATE.md

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+*Description*:
+*Testing*:
+*Documentation*:
+[Optional Fixes #issue]
+
+Please see a detailed explanation of how to fill out the fields in the relevant sections in PULL_REQUEST.md.