Add 'contrib' directory (#1333)

Summary:
## Motivation and Context

We need the 'contrib' directory to hold experimental code, which is accessible in OSS. First 2 use cases: 1) moving HuggingFace transform related code and dependency there 2) maintain prototypes for PyText Library there

## How Has This Been Tested

## Types of changes

- [x] Docs change / refactoring / dependency upgrade
- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing functionality to change)

## Checklist

- [ ] My code follows the code style of this project.
- [x] My change requires a change to the documentation.
- [x] I have updated the documentation accordingly.
- [ ] I have read the **CONTRIBUTING** document.
- [x] I have completed my CLA (see **CONTRIBUTING**)
- [ ] I have added tests to cover my changes.
- [ ] All new and existing tests passed.
Pull Request resolved: #1333

Reviewed By: snisarg

Differential Revision: D21189667

Pulled By: hudeven

fbshipit-source-id: fa01a03adfd9413adbc7342c3ba5628f3226fdf5

Author: hudeven

pytext/contrib/README.md

@@ -0,0 +1,14 @@
+# What is 'contrib' directory?
+
+Code in 'contrib' directory is considered experimental. If it's proved to be useful, we may incorporate it into the core PyText structure. The code review turnaround time for changes in 'contrib' is expected to be shorter than changes in the core PyText structure.
+
+'contrib' is organized by projects. We recommend the following directory structure to prevent file collisions:
+
+```
+  contrib/
+    my_project/     - project name
+      common/       - mimic the core PyText structure
+      config/            
+      data/
+      ...
+```