Create Method and Features

Also precise the usage of the hash function (see review pierrepeterlongo/back_to_sequences#6 (comment))

docs/source/Method and Features

@@ -0,0 +1,38 @@
+
+`back_to_sequences` is written in `rust`. It uses the native HashMap for storing the searched $k$-mer set,
+with alternative [aHash](https://github.com/tkaitchuck/aHash) hash function. This data structure is used to index each of the searched $k$-mer set. 
+
+Depending on the user choice, the original or the canonical version of
+each $k$-mer from the "reference-$k$-mers" set is indexed. The source code
+is unit-tested and functionally tested using tools from the Rust community.
+
+At query time, given a sequence $s$ from $\mathcal{S}$, all its $k$-mers
+are extracted and queried using the HashMap. Depending on the user's choice, the canonical
+or the original representation of each $k$-mer from the reference set is
+indexed. Again, depending on the user's choice, for each queried
+$k$-mer, the original version, its reverse complement, or its canonical
+representation is queried. If the queried $k$-mer belongs to the index,
+its number of occurrences is increased. The number of $k$-mers extracted
+from the sequence $s$ that have a match with the index is output
+together with the original sequence. As an option, in addition to only
+counting the number of occurrences of each $k$-mer from $\mathcal{K}$ in
+$\mathcal{S}$, `back_to_sequences` enables to also record their
+occurrence positions and orientation and to output this information.
+Sequences are queried in parallel.
+
+A minimal and a maximal threshold can be fixed by the user. A queried
+sequence is output only if its percentage of $k$-mers that belong to the
+searched $k$-mers is strictly higher than the minimal threshold and
+lower or equal to the maximal threshold. While the minimal threshold
+enables to focus on sequences that are similar enough with a set of
+$k$-mers, the maximal threshold offers for instance a way to remove
+contaminated sequences.
+
+The sequences to be queried can be provided as a fasta or fastq file
+(gzipped or not). They can also be read directly from the standard
+input (*stdin*). This offers the may to stream sequences as they arrive,
+for instance when they are obtained during an Oxford Nanopore sequencing
+process.
+
+The output format of queried sequences respects the input format: if the
+input is in fasta (resp. fastq), the output is in fasta (resp. fastq).