Docs: add the contributor guide

Author: ForNeVeR

CONTRIBUTING.md

@@ -0,0 +1,25 @@
+Contributor Guide
+=================
+
+Build
+-----
+
+> **Note**
+> 
+> This project uses source generators that are not included into the solution file. This may cause problems if you build the project in IDE.
+> 
+> If you experience any problems mentioning the `FixedMath.NET.Generators` assembly, run the following shell command before opening the project in the IDE:
+> 
+> ```console
+> $ dotnet build src/FixedMath.NET.Generators
+> ```
+> 
+> (alternately, open said project in the IDE and build it from there)
+
+Before building the project in the IDE, remember to build the source generators:
+
+To build the project, run this shell command:
+
+```
+$ dotnet build
+```

README.md

@@ -18,3 +18,7 @@ In the unit tests you'll find implementations for Int32-based (Q15.16) and Byte-
 This project started as a port of libfixmath (http://code.google.com/p/libfixmath/).
 
 Note that the type requires explicit casts to convert to floating point and this is intentional, the difference between fixed point and floating point math is as important as the one between floating point and integral math.
+
+## Documentation
+
+- [Contributor Guide](CONTRIBUTING.md)