Add asn1grep tool

Scrolling through the ASN.1 files for datatypes is a nightmare, so I
wrote a script to automate that

tools/README.md

@@ -0,0 +1,16 @@
+## Rayhunter tools
+
+### `asn1grep.py`: a script for finding a datatype in ASN.1 files
+
+`asn1grep` parses our ASN.1 spec files, then searches for a given datatype by recursively descending through the LTE-RRC types we care about. it then prints out each result as a "path" through the highly nested datatypes.
+
+Setup:
+1. `python -m venv .venv && . .venv/bin/activate`
+2. `pip install -r requirements.txt`
+
+Usage:
+```
+» python asn1grep.py IMSI                                  
+searching for IMSI
+PCCH-Message [message [message.c1 [c1 [c1.paging [paging [pagingRecordList[0] [ [ue-Identity [ue-Identity.imsi [IMSI]]]]]]]]]]
+```