Create terms glossary for sourced.ml

[Konstantin Slavnov]

We constantly confuse terms, so what to say about other developers.
I do not want to make it full, but to have a start.

Here is terms list to explain on the first iteration:

1. Bag-of-words
2. Weighted bag-of-words
3. Model
4. Algorithm
5. Transformer
6. Document
7. Features
   1. identifier
   2. token
   3. literal
   4. graphlet

Googleable terms we may comment:

1. quantization
2. TF-IDF
3. topic
4. co-occurrence matrix

@src-d/machine-learning please take a look and add any confusing terms you remember.

[Romain Keramitas]

If we're gonna define identifiers and token, might as well also add literals, graphlets and also quantification quantization . I think we could divide the glossary into:

• terms that mean something more specific then would be usually the case or are vague to start with e.g. model meaning a modelforge model, words in BOW being any feature extracted from a document, document that means a repo/file or function, etc.
• terms that we use in the same ways it is intended but not be well known. Now of course they have Google, but we might as well drop a couple lines to explain the concept. E.g. COOC, quantization, topics, TFIDDF

[Vadim Markovtsev]

Linking to https://github.com/src-d/apollo/blob/master/doc/GLOSSARY.md

[Konstantin Slavnov]

Thanks, @r0mainK I update the description.