Added the pronunciation.txt sample file for en-US language (#488)

* Added sample data for custom speech.

* Added the readme for sample data

* Updated the git attribute files

* Added spaces for related text file

* Removed the additional space

* Added the link from home readme to sample data custome speech readme

* Updated the related text to make the sample data better for training.

* Add the pronunciation.txt file for en-US

sampledata/customspeech/en-US/training/pronunciation.txt

@@ -0,0 +1,3 @@
+3CPO	three c p o
+CNTK	c n t k
+IEEE	i triple e