feat: Add how-to guide

Author: MrHedmad

HOW_TO.md

@@ -0,0 +1,14 @@
+# How to
+
+This page compiles common operations that you might want to do when using the MTP-DB.
+
+## How to find all GAP junctions?
+This SQL gets you all the GAP junction ENSGs. They are best detected by their gene name, as the TCDB families seem to be incomplete:
+```sql
+SELECT ensg
+FROM gene_names
+WHERE
+	hugo_gene_symbol like "GJ%" OR
+	hugo_gene_symbol like "PANX%"
+```
+

README.md

@@ -13,6 +13,7 @@ Daedalus takes care of connecting to, downloading from, parsing and patching dat
 You can take a look at the [CHANGELOG.md](CHANGELOG.md) file for a changelog of each release of the database.
 
 To browse the database locally, we suggest SQLite browsers such as [this FOSS program](https://sqlitebrowser.org/).
+Common SQL queries to solve some routine problems are detailed in the [How to guide](HOW_TO.md).
 
 The structure of the MTP-DB is detailed [in this excel file](https://docs.google.com/spreadsheets/d/16vb_2FvZdaUSVOc8m9yRPT6eYP8cGl4UsOzauI2Qbek/edit?usp=sharing).