Make it easier to discover the (excellent) developer docs

As mentioned in #253, it's a real shame to have such great developer
docs which are "hidden" by not being conspicuously linked from
anywhere.

So link to them from the top-level README.md, and also add a
CONTRIBUTING.md which is a conventional file location understood by
sites such as GitHub:

  https://github.com/open-source-labs/reactime/community
  https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/setting-guidelines-for-repository-contributors#adding-a-contributing-file

Partially addresses #253.

Author: Adam Spiers

CONTRIBUTING.md

@@ -0,0 +1,8 @@
+# Contributing to Reactime
+
+Thanks very much for your interest in contributing to this project!
+
+Reactime is an open source project, and we’d really appreciate your
+help with improving user experience. Please read [the developer
+README](src/README.md), and create a pull request (or issue) to
+propose and collaborate on changes to a repository.

README.md

@@ -81,7 +81,7 @@ Try refreshing the application you want to test and refresh the DevTools by clic
 
 ### ❓ <b>I found a bug in Reactime</b>
 
-Reactime is an open source project, and we’d really appreciate your help with improving user experience. Please create a pull request (or issue) to propose and collaborate on changes to a repository.
+Reactime is an open source project, and we’d really appreciate your help with improving user experience. Please read [the developer README](src/README.md), and create a pull request (or issue) to propose and collaborate on changes to a repository.
 
 ## <b>Features</b>