Add hints about coding style

heavily borrowed from osm2pgsql

CONTRIBUTING.md

@@ -0,0 +1,47 @@
+# Nominatim contribution guidelines
+
+## Workflow
+
+We operate the "Fork & Pull" model explained at
+
+https://help.github.com/articles/using-pull-requests
+
+You should fork the project into your own repo, create a topic branch
+there and then make one or more pull requests back to the openstreetmap repository.
+Your pull requests will then be reviewed and discussed.
+
+## Coding style
+
+Nominatim historically hasn't followed a particular coding style but we
+are in process of consolodating the style. The following rules apply:
+
+ * Python code uses the official Python style
+ * indention
+   * SQL use 2 spaces
+   * all other use files TABs
+   * [BSD style](https://en.wikipedia.org/wiki/Indent_style#Allman_style) for braces
+ * spaces
+   * spaces before and after equal signs and operators
+   * no trailing spaces
+   * no spaces after opening and before closing bracket
+   * leave out space between a function name and bracket
+     but add one between control statement(if, while, etc.) and bracket
+
+
+This coding style must be applied to any new or changed code. You are also
+welcome to fix the coding style of existing code but please submit separate
+PRs for this.
+
+## Testing
+
+Before submitting a pull request make sure that the following tests pass:
+
+```
+  cd tests
+  NOMINATIM_DIR=<builddir> lettuce -t -Fail features/db features/osm2pgsql
+```
+
+```
+  cd test-php
+  phpunit ./
+```