add start of security notes

security.md

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+# Introduction
+
+# Local Filesystem
+
+The main use of the local file system for the core library (other than the validator - see below) is for the 
+[NPM package cache](). The default location and content is as specified in the FHIR specification, bu you 
+can choose where this goes if you want, or provide your own NPM package cache manager
+
+However there are other uses of the local file system scattered throughout the code. Long term, 
+the intention is to clean this up so these are all following a consistent policy.
+
+All access to the local file system runs through the class ManagedFileAccess. You can 
+set the static features of this class to completely cut the library off from the 
+local filesystem, or provide your own file system accessor, or limit the files accessed
+to particular sub-directories. See ManagedFileAccess for details.
+
+# Network access
+
+The library will access the web to download needed collateral, or to access terminology resources or servers.
+All access is by http(s) using the Apache... library, and is controlled by the class XXXX where you can 
+turn all network access off.
+
+# Logging 
+
+todo
+
+# Cryptography 
+
+Other than the https client, the library doesn't have any crypto functions in it. 
+
+TODO: Actually, it does, reading SHCs 
+